Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 28, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	Editas Medicine, Inc.
Entity Central Index Key	0001650664
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 502,961,068
Entity Common Stock, Shares Outstanding		46,884,857
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 146,630	$ 185,323
Marketable securities	182,509
Accounts receivable	679	88
Prepaid expenses and other current assets	2,381	1,772
Total current assets	332,199	187,183
Property and equipment, net	39,442	40,378
Restricted cash and other non-current assets	1,619	1,621
Total assets	373,260	229,182
Current liabilities:
Accounts payable	4,020	4,640
Accrued expenses	11,049	17,439
Notes payable	7,500	10,000
Deferred revenue, current	13,238	256
Other current liabilities	900	748
Total current liabilities	36,707	33,083
Deferred revenue, net of current portion	94,725	26,000
Construction financing lease obligation, net of current portion	33,431	35,096
Other non-current liabilities	317	396
Total liabilities	165,180	94,575
Commitments and contingencies (see note 8)
Stockholders’ equity
Preferred stock, $0.0001 par value per share: 5,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.0001 par value per share: 195,000,000 shares authorized; 45,025,448 and 36,662,724 shares issued, and 44,507,960 and 35,818,131 shares outstanding at December 31, 2017 and December 31, 2016, respectively	4	4
Additional paid-in capital	514,002	320,129
Accumulated other comprehensive loss	(76)
Accumulated deficit	(305,850)	(185,526)
Total stockholders’ equity	208,080	134,607
Total liabilities and stockholders’ equity	$ 373,260	$ 229,182

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Consolidated Balance Sheet
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	195,000,000	195,000,000
Common stock, shares issued	45,025,448	36,662,724
Common stock, shares outstanding	44,507,960	35,818,131

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Operations
Collaboration and other research and development revenues	$ 13,728	$ 6,053	$ 1,629
Operating expenses:
Research and development	83,159	56,979	18,846
General and administrative	50,502	46,262	18,095
Total operating expenses	133,661	103,241	36,941
Operating loss	(119,933)	(97,188)	(35,312)
Other income (expense), net:
Other income (expense), net	587	(57)	(37,445)
Interest (expense) income, net	(978)	62	(143)
Total other (expense) income, net	(391)	5	(37,588)
Net loss	(120,324)	(97,183)	(72,900)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(120,324)	(97,183)	(72,900)
Accretion of redeemable convertible preferred stock to redemption value		(47)	(394)
Net loss attributable to common stockholders	$ (120,324)	$ (97,230)	$ (73,294)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.98)	$ (3.02)	$ (28.55)
Weighted-average common shares outstanding, basic and diluted	40,323,631	32,219,717	2,566,916

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Comprehensive Loss
Net loss	$ (120,324)	$ (97,183)	$ (72,900)
Other comprehensive loss:
Unrealized loss on marketable securities	(76)
Comprehensive loss	$ (120,400)	$ (97,183)	$ (72,900)

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Redeemable Convertible Preferred Stock.	Series A-2	Series B redeemable convertible	Common Stock. Follow-on Offering	Common Stock. Public Offering	Common Stock.	Additional Paid-In Capital Follow-on Offering	Additional Paid-In Capital Public Offering	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Follow-on Offering	Public Offering	Total
Balance, beginning of period at Dec. 31, 2014	$ 20,772
Balance, beginning of period (in shares) at Dec. 31, 2014	21,260,000
Statements of Redeemable Convertible Preferred Stock
Issuance of redeemable convertible preferred stock, net of issuance costs		$ 21,989	$ 119,722
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)		16,890,699	26,666,660
Reclassification of tranche rights upon issuance of redeemable convertible preferred stock	$ 37,038													$ 37,038
Accretion of redeemable convertible preferred stock to redemption value	394													394
Balance, end of period at Dec. 31, 2015	$ 199,915
Balance, end of period (in shares) at Dec. 31, 2015	64,817,359
Balance, beginning of period at Dec. 31, 2014									$ 156	$ (15,448)				(15,292)
Balance, beginning of period (in shares) at Dec. 31, 2014						1,863,169
Statement of Stockholders' Deficit
Accretion of redeemable convertible preferred stock to redemption value									(394)					(394)
Issuance of common stock to licensors upon settlement of antidilution protection liability									1,936					1,936
Issuance of common stock to licensors upon settlement of antidilution protection liability (in shares)						327,970
Exercise of stock options									6					6
Exercise of stock options (in shares)						28,651
Vesting of restricted common stock and common stock subject to repurchase									17					17
Vesting of restricted common stock and common stock subject to repurchase (in shares)						653,272
Vesting of founders shares									2,345					2,345
Vesting of founders shares (in shares)						360,576
Stock-based compensation expense									1,168					1,168
Net loss										(72,900)				(72,900)
Balance, end of period at Dec. 31, 2015									5,234	(88,348)				(83,114)
Balance, end of period (in shares) at Dec. 31, 2015						3,233,638
Statements of Redeemable Convertible Preferred Stock
Accretion of redeemable convertible preferred stock to redemption value	$ 47													47
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering	$ (199,962)
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering (in shares)	(64,817,359)
Statement of Stockholders' Deficit
Accretion of redeemable convertible preferred stock to redemption value									(47)					(47)
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering						$ 3			199,954	5				199,962
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering (in shares)						24,929,709
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering									376					376
Issuance of common stock from public offering, net of issuance costs						$ 1			97,487					97,488
Issuance of common stock from public offering, net of issuance costs (in shares)						6,785,000
Exercise of common stock warrant (in shares)						19,271
Exercise of stock options									233					233
Exercise of stock options (in shares)						58,915
Vesting of restricted common stock and common stock subject to repurchase									11					11
Vesting of restricted common stock and common stock subject to repurchase (in shares)						431,018
Vesting of founders shares									8,315					8,315
Vesting of founders shares (in shares)						360,580
Stock-based compensation expense									8,566					8,566
Net loss										(97,183)				(97,183)
Balance, end of period at Dec. 31, 2016						$ 4			320,129	(185,526)				134,607
Balance, end of period (in shares) at Dec. 31, 2016						35,818,131
Statement of Stockholders' Deficit
Issuance of common stock from public offering, net of issuance costs							$ 96,685	$ 57,223				$ 96,685	$ 57,223
Issuance of common stock from public offering, net of issuance costs (in shares)				4,600,000	2,265,500
Issuance of common stock for repayment of notes payable									14,823					14,823
Issuance of common stock for repayment of notes payable (in shares)						750,617
Exercise of stock options									1,768					1,768
Exercise of stock options (in shares)						272,210
Vesting of restricted common stock and common stock subject to repurchase									4,096					4,096
Vesting of restricted common stock and common stock subject to repurchase (in shares)						561,118
Vesting of founders shares									3,989					3,989
Vesting of founders shares (in shares)						240,384
Stock-based compensation expense									15,289					15,289
Unrealized losses on marketable securities											$ (76)			(76)
Net loss										(120,324)				(120,324)
Balance, end of period at Dec. 31, 2017						$ 4			$ 514,002	$ (305,850)	$ (76)			$ 208,080
Balance, end of period (in shares) at Dec. 31, 2017						44,507,960

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock issuance costs			$ 1,746
Common Stock.
Stock issuance costs		$ 11,100
Common Stock. \| Follow-on Offering
Stock issuance costs	$ 600
Common Stock. \| Public Offering
Stock issuance costs	$ 1,700
Series A-2
Stock issuance costs			1
Series B redeemable convertible
Stock issuance costs			$ 300

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flow from operating activities
Net loss	$ (120,324)	$ (97,183)	$ (72,900)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	23,364	16,881	3,513
Depreciation	2,683	1,202	471
Non-cash research and development expense	14,500	10,000
Re-measurement of warrant to purchase redeemable securities		87	241
Change in fair value of preferred stock tranche asset or liability			35,551
Changes in fair value of anti-dilutive protection liability			1,609
Other non-cash items, net	(300)	869	53
Changes in operating assets and liabilities:
Accounts receivable	(591)	931	(1,019)
Prepaid expenses and other current assets	(596)	(1,306)	(373)
Other non-current assets	2	2,246
Accounts payable	(1,515)	3,251	(1,436)
Accrued expenses	(8,334)	11,841	3,526
Deferred revenue	81,707	935	25,321
Other current and non-current liabilities	(13)
Net cash used in operating activities	(9,417)	(50,246)	(5,443)
Cash flow from investing activities
Purchases of property and equipment	(2,059)	(3,493)	(1,431)
Proceeds from the sale of equipment	15	20
Purchases of marketable securities	(375,266)
Proceeds from maturities of marketable securities	193,500
Net cash used in investing activities	(183,810)	(3,473)	(1,431)
Cash flow from financing activities
Proceeds from equipment loan, net of issuance costs			1,500
Proceeds from public offering of common stock, net of issuance costs	154,143	97,488
Proceeds from exercise of stock options	1,755	233
Payments on construction financing lease obligation	(764)	(560)
Proceeds from the issuance of redeemable convertible preferred stock and tranche rights, net of issuance costs			141,711
Payments of notes payable	(600)
Payments of equipment loan principal			(2,000)
Payments of final fee for loan payoff			(80)
Proceeds from the issuance of common stock and restricted stock			6
Payments of initial public offering costs			(1,746)
Net cash provided by financing activities	154,534	97,161	139,391
Net increase (decrease) in cash, cash equivalents, and restricted cash	(38,693)	43,442	132,517
Cash, cash equivalents, and restricted cash, beginning of period	186,942	143,500	10,983
Cash, cash equivalents, and restricted cash, end of period	148,249	186,942	143,500
Supplemental disclosure of cash and non-cash activities:
Accretion of redeemable convertible preferred stock to redemption value		47	394
Fixed asset additions included in accounts payable and accrued expenses	623	130	58
Construction financing lease obligation		35,941
Conversion of anti-dilutive protection liability to common stock			1,936
Reclassification of warrants to additional paid in capital		376
Conversion of preferred stock to common stock upon closing of the initial public offering		199,962
Reclassification of liability for common stock subject to repurchase	11	11	17
Interest paid	13	$ 465	91
Offering expenses included in accounts payable and accrued expenses	235		502
Reclassification of tranche rights upon issuance of redeemable convertible preferred stock			$ 37,038
Issuance of common stock for settlement of notes payable	$ 14,823

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2017
Nature of Business
Nature of Business	Editas Medicine, Inc. Notes to Consolidated Financial Statements 1. Nature of Business Editas Medicine, Inc. (the “Company”) is a research stage company dedicated to treating patients with genetically defined diseases by correcting their disease‑causing genes. The Company was incorporated in the state of Delaware in September 2013. Its principal offices are in Cambridge, Massachusetts. Since its inception, the Company has devoted substantially all of its efforts to business planning, research and development, recruiting management and technical staff, and raising capital. The Company has primarily financed its operations through various equity and debt financings, including the initial public offering of its common stock (the “IPO”), its follow-on public offerings of its common stock in March 2017 and December 2017, and private placements of preferred stock, from upfront, milestone and research and development fees paid under a research collaboration with Juno Therapeutics, Inc. (“Juno Therapeutics”), and from an upfront payment paid under a strategic alliance and option agreement with Allergan Pharmaceuticals International Limited (“Allergan”). The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations and ability to transition from pilot‑scale manufacturing to large‑scale production of products. Liquidity In February 2016, the Company completed its IPO whereby the Company sold 6,785,000 shares of its common stock, inclusive of 885,000 shares of common stock sold by the Company pursuant to the full exercise of an option granted to the underwriters in connection with the offering, at a price to the public of $16.00 per share. The shares began trading on the Nasdaq Global Select Market on February 3, 2016. The aggregate net proceeds received by the Company from the offering were $97.5 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company. In March 2017, the Company completed a follow-on offering whereby the Company sold 4,600,000 shares of its common stock, inclusive of 600,000 shares of common stock sold by the Company pursuant to the full exercise of an option granted to the underwriters in connection with the offering, at a price to the public of $22.50 per share (the “March Offering”). The aggregate net proceeds received by the Company from the March Offering were $96.7 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company. In December 2017, the Company completed a public offering whereby the Company sold 2,265,500 shares of its common stock, inclusive of 295,500 shares of common stock sold by the Company pursuant to the full exercise of an option granted to the underwriter in connection with the offering, at a price to the public of $26.00 per share (the “December Offering”). The aggregate net proceeds received by the Company from the December Offering were $57.2 million, after deducting underwriting discounts and other offering expenses payable by the Company. The Company has incurred annual net operating losses in every year since its inception. The Company expects that its existing cash, cash equivalents, and marketable securities at December 31, 2017, anticipated interest income, and anticipated research support under the Company’s collaboration agreement with Juno Therapeutics will enable it to fund its operating expenses and capital expenditure requirements for at least the next 24 months following the date of this Annual Report on Form 10-K. The Company had an accumulated deficit of $305.9 million at December 31, 2017, and will require substantial additional capital to fund its operations. The Company has never generated any product revenue. There can be no assurance that the Company will be able to obtain additional debt or equity financing or generate product revenue or revenues from collaborative partners, on terms acceptable to the Company, on a timely basis or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of significant accounting policies

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of Editas Medicine, Inc. and its wholly owned subsidiary, Editas Securities Corporation, which is a Delaware subsidiary created to buy, sell and hold securities. All intercompany transactions and balances have been eliminated.

Basis of Presentation

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Reclassification

Certain prior period amounts have been reclassified for consistency with the current period presentation. These reclassifications had no effect on previously reported results of operations.

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, estimates related to revenue recognition, accrued expenses, stock-based compensation expense, deferred tax valuation allowances and sub-license fees due to certain of its licensors. The Company bases its estimates on historical experience and other market-specific or relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions.

Fair Value of Financial Instruments

ASC Topic 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances.

ASC 820 identifies fair value as the exchange price representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three‑tier fair value hierarchy that distinguishes between the following:

Level 1 – Quoted market prices in active markets for identical assets or liabilities.

Level 2 – Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves.

Level 3 – Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The carrying amounts reflected in the consolidated balance sheets for cash and cash equivalents, restricted cash, marketable securities, accounts receivable, prepaid expenses and other current assets, accounts payable, accrued expenses, and other current liabilities approximate their fair values, due to their short‑term nature. The Company believes that the carrying value of the notes payable approximates their fair value based on Level 3 inputs including a quoted rate.

Cash, Cash Equivalents, and Restricted Cash

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in money market funds and U.S. government-backed securities.

The Company had restricted cash of $1.6 million, $1.6 million, and $0.3 million held in the form of money market accounts as collateral for the Company’s facility lease obligation as of December 31, 2017, 2016 and 2015, respectively.

The following table presents cash, cash equivalents, and restricted cash as reported on the consolidated balance sheets that equal the total amounts on the consolidated statements of cash flows (in thousands):


	Year Ended
	As of December 31,
	2017		2016		2015
Cash and cash equivalents	$	146,630	$	185,323	$	143,180
Restricted cash included in "Prepaid expenses and other current assets"		—		—		320
Restricted cash included in "Restricted cash and other non-current assets"		1,619		1,619		—
Total	$	148,249	$	186,942	$	143,500

Marketable Securities

The Company classifies marketable securities with a remaining maturity when purchased of greater than three months and less than one year from the balance sheet date as current. Marketable securities with a remaining maturity date greater than one year are classified as non-current. The Company classifies all of its marketable securities as available-for-sale securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive loss as a component of stockholders’ equity (deficit) until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the of the underlying security. Realized gains and losses are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary.” To determine whether an impairment is other-than-temporary, the Company considers whether it has the ability and intent to hold the investment until a market price recovery and considers whether evidence indicating the cost of the investment is recoverable outweighs evidence to the contrary. The Company does not intend to sell the investments and it is not likely that the Company will be required to sell the investments before recovery of their amortized cost bases, which may be maturity.

Accounts Receivable

The Company makes judgments as to its ability to collect outstanding receivables and provides an allowance for receivables when collection becomes doubtful. Provisions are made based upon a specific review of all significant outstanding invoices and the overall quality and age of those invoices not specifically reviewed. The Company's receivables primarily relate to amounts reimbursed under its collaboration agreement with Juno Therapeutics. The Company believes that credit risks associated with its collaborations partner is not significant. To date, the Company has not had any write-offs of bad debt, and the Company did not have an allowance for doubtful accounts as of December 31, 2017 and 2016.

Property and Equipment

Property and equipment consists of computers, laboratory equipment, furniture and office equipment, and leasehold improvements and is stated at cost, less accumulated depreciation. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred, while costs of major additions and betterments are capitalized. Depreciation is calculated over the estimated useful lives of the assets using the straight‑line method. The Company capitalizes laboratory equipment used for research and development if it has alternative future use in research and development or otherwise.


Asset:		Estimated Useful life
Lab equipment		5 years
Computer equipment and software		3 years
Furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or remaining lease term
Building		30 years

The Company records certain estimated costs incurred and reported by a landlord as an asset and corresponding financing lease obligation on the consolidated balance sheets. See Note 8, “Commitments and contingencies,” for additional information.

Impairment of Long‑lived Assets

The Company evaluates long‑lived assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2017.

Revenue Recognition

To date, the Company has primarily earned revenue under the collaboration and license agreement with Juno Therapeutics and the strategic research alliance with Allergan (see Note 9).

The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition (“ASC 605”). Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met:

Persuasive evidence of an arrangement exists;

Delivery has occurred or services have been rendered;

The seller’s price to the buyer is fixed or determinable; and

Collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified in current liabilities. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

The Company evaluates multiple‑element arrangements based on the guidance in ASC Topic 605‑25, Revenue Recognition Multiple‑Element Arrangements (“ASC 605‑25”). Pursuant to the guidance in ASC 605‑25, the Company evaluates multiple‑element arrangements to determine (1) the deliverables included in the arrangement and (2) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. This evaluation involves subjective determinations and requires the Company to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that the delivered item has value to the customer on a standalone basis and, if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use a deliverable for its intended purpose without the receipt of the remaining deliverable, whether the value of the deliverable is dependent on the undelivered item and whether there are other vendors that can provide the undelivered items.

Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the cost to exercise the option, the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option and the likelihood the option will be exercised. When an option is considered substantive, the Company does not consider the option or item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable consideration, assuming the option is not priced at a significant and incremental discount. Conversely, when an option is not considered substantive, the Company would consider the option, including other deliverables contingent upon the exercise of the option, to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration. In addition, if the price of the option includes a significant incremental discount, the discount would be included as a deliverable at the inception of the arrangement.

The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting using the relative selling price method. The Company determines the estimated selling price for units of accounting within each arrangement using vendor‑specific objective evidence (“VSOE”) of selling price, if available, third‑party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price (“BESP”) if neither VSOE nor TPE is available. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity‑specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting.

The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the Company’s contractual or estimated performance period for the undelivered elements, which is typically the term of the Company’s research and development obligations. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight‑line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight‑line method or proportional performance method, as applicable, as of the period ending date.

At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (1) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from its performance to achieve the milestone, (2) the consideration relates solely to past performance and (3) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone and the level of effort and investment required to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, assuming all other revenue recognition criteria are met.

The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met.

Research and Development Costs

Research and development costs are charged to expense as incurred in performing research and development activities. The costs include employee‑related expenses including salaries, benefits, and stock‑based compensation expense, costs of funding research performed by third parties that conduct research and development and preclinical activities on the Company’s behalf, the cost of purchasing lab supplies and non‑capital equipment used in preclinical activities and in manufacturing preclinical study materials, consultant fees, facility costs including rent, depreciation, and maintenance expenses, and fees for acquiring and maintaining licenses under third party licensing agreements, including any sublicensing or success payments made to the Company’s licensors. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from the Company’s estimate, the accrual or prepaid is adjusted accordingly. The Company defers and capitalizes non-refundable advance payments made by the Company for research and development activities until the related goods are received or the related services are performed. In circumstances where amounts have been paid in excess of costs incurred, the Company records a prepaid expense.

Patent Costs

The Company expenses patent and patent application costs and related legal costs for the prosecution and maintenance of such patents and patent applications, including patents and patent applications the Company licenses, as incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations.

Construction Financing Lease Obligation

Beginning in 2016, the Company began recording certain estimated construction costs incurred and reported to the Company by a landlord as an asset and corresponding construction financing lease obligation on the Company’s consolidated balance sheets because it was deemed to be the owner of the building during the construction period for accounting purposes. In each reporting period, the landlord estimated and reported to the Company the costs incurred to date and provided supporting invoices for the Company to review. The Company periodically met with the landlord and its construction manager to review the estimates and observe construction progress prior to recording such amounts. Construction was completed in October 2016 and the Company considered the requirements for sale-leaseback accounting treatment, which included an evaluation of whether all risks of ownership had transferred back to the landlord as evidenced by a lack of continuing involvement in the lease property. The Company determined that the arrangement did not qualify for sale lease-back accounting treatment, the building asset will remain on the Company’s balance sheet at its historical cost, and such asset would be depreciated over its estimated useful life of thirty years.

Stock‑based Compensation Expense

The Company accounts for stock-based compensation awards in accordance with ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all share-based payments to employees, including grants of employee stock options, to be recognized as expense in the consolidated statements of operations based on their grant date fair values. For stock options granted to employees and to members of the Company’s board of directors for their services on the board of directors, the Company estimates the grant date fair value of each option award using the Black-Scholes option-pricing model. For stock options subject to service-based vesting conditions, the Company recognizes stock-based compensation expense equal to the grant date fair value of stock options on a straight-line basis over the requisite service period.

Share-based payments issued to non-employees are initially recorded at their fair values, and are revalued at each reporting date and as the equity instruments vest and are recognized as expense over the related service period in accordance with the provisions of ASC Topic 505-50, Equity-Based Payments to Non-Employees.

The Black‑Scholes option pricing model requires the input of certain subjective assumptions, including (1) the expected stock price volatility, (2) the calculation of expected term of the award, (3) the risk‑free interest rate, and (4) the expected dividend yield. Because there had been no public market for the Company’s common stock prior to the IPO, there was a lack of company‑specific historical and implied volatility data. Accordingly, the Company bases its estimates of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The Company calculates historical volatility based on a period of time commensurate with the expected term. The Company computes expected volatility based on the historical volatility of a representative group of companies with similar characteristics to the Company, including their stages of product development and focus on the life science industry. The Company uses the simplified method as prescribed by the Securities and Exchange Commission’s Staff Accounting Bulletin No. 107, Share‑Based Payment, to calculate the expected term for options granted to employees as the Company does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For options granted to non‑employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term. The Company determines the risk‑free interest rate based on a treasury instrument whose term is consistent with the expected term of the stock options. The Company uses an assumed dividend yield of zero as the Company has never paid dividends and does not have current plans to pay any dividends on common stock. If factors change or different assumptions are used, the Company’s stock-based compensation expense could be materially different in the future.

Determination of Fair Value of Common Stock on Grant Dates prior to our Initial Public Offering

Prior to the IPO, the Company utilized significant estimates and assumptions in determining the fair value of its common stock. The board of directors determined the estimated fair value of the Company’s common stock based on a number of objective and subjective factors, including the lack of an active public market for the Company’s common and convertible preferred stock; the prices of shares of the Company’s convertible preferred stock that the Company had sold to outside investors in arm’s length transactions, and the rights, preferences, and privileges of that convertible preferred stock relative to the Company’s common stock; the Company’s results of operations and financial condition; the Company’s entry into license agreements, pursuant to which the Company obtained rights to important intellectual property; the material risks related to the Company’s business; the Company’s business strategy; the market performance of publicly traded companies in the life sciences and biotechnology sectors; and the likelihood of achieving a liquidity event for the holders of the Company’s common stock, such as an initial public offering, given prevailing market conditions. The Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants, Audit and Accounting Practice Aid Series: Valuation of Privately Held Company Equity Securities Issued as Compensation (the “AICPA Practice Aid”), to estimate the fair value of its common stock and in performing retrospective valuation analyses for certain grant dates prior to the IPO. The methodologies included the option pricing method utilizing the back-solve method (a form of the market approach defined in the AICPA Practice Aid) and the probability-weighted expected return method based upon the probability of occurrence of certain future liquidity events such as an initial public offering or sale of the Company. Each valuation methodology included estimates and assumptions that required the Company’s judgment. Significant changes to the key assumptions used in the valuations could have resulted in different fair values of the Company’s common stock at each valuation date.

Income taxes

Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial reporting and the tax reporting basis of assets and liabilities and are measured using the enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the weight of available evidence, it is more likely than not that the deferred tax assets will be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognized the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances.

Other Income (Expense), Net

Other income (expense), net consists primarily of interest income earned on cash equivalents and marketable securities, interest expense on the construction financing lease obligation and promissory notes, rental income from the Company’s subtenant, interest income, accretion of discounts, and amortization of premiums associated with marketable securities.

Prior to 2017, other income (expense), net consisted primarily of interest income earned on cash equivalents and government grant income, net of re-measurement losses associated with changes in the fair value of the Company’s liability for a warrant to purchase preferred stock. Upon the completion of the IPO, the outstanding warrant to purchase preferred stock converted into a warrant to purchase common stock and the Company reclassified the fair value of the warrant to additional paid-in capital. As a result, there were no further remeasurement gains or losses associated with the warrant after the first quarter of 2016.

Comprehensive Loss

Comprehensive loss is comprised of net loss and other comprehensive income or loss. Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. Comprehensive loss currently consists of net loss and changes in unrealized losses on marketable securities.

Concentrations of Credit Risk and Off‑Balance Sheet Risk

The Company has no financial instruments with off‑balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially subject the Company to a concentration of credit risk are cash, cash equivalents, marketable securities and accounts receivable. The Company’s cash, cash equivalents and marketable securities are held in accounts at a financial institution that may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds. Accounts receivable primarily consist of amounts due under the collaboration agreement with Juno Therapeutics (see Note 9) for which the Company does not obtain collateral. As of December 31, 2017, substantially all of the Company’s revenue to date has been generated from the strategic alliance with Allergan and the collaboration with Juno Therapeutics.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, view the Company’s operations and manage the Company’s business as a single operating segment, which is the business of developing and commercializing genome editing technology.

Recent Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition, and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The update also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. ASU 2014-09 is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017 and should be applied retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying this update recognized at the date of initial application. In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing (“ASU 2016-10”), which clarifies certain aspects of identifying performance obligations and licensing implementation guidance. In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients (“ASU 2016-12”), related to disclosures of remaining performance obligations, as well as other amendments to guidance on collectability, non-cash consideration and the presentation of sales and other similar taxes collected from customers. These standards have the same effective date and transition date as ASU 2014-09. The Company has three revenue arrangements, its license and collaboration with Juno Therapeutics, its award arrangement with the Cystic Fibrosis Foundation Therapeutics, Inc. (“CFFT”), and its strategic alliance with Allergan, pursuant to which it has recognized since inception a total of $12.2 million, $0.3 million, and $8.8 million, respectively, through December 31, 2017. The Company is analyzing the potential impact that ASU 2014-09, ASU 2016-10 and ASU 2016-12 may have on its historical revenue recognition under these three arrangements. This analysis includes, but is not limited to, reviewing variable consideration as it relates to its agreements and in particular, milestone payments as the inclusion of milestone payments in the transaction price could accelerate revenue recognized under ASC 606 compared to ASC 605, evaluating whether a significant financing component is present, determining the revenue recognition method for services performed under the arrangement, and assessing potential disclosures and evaluating the impact of each potential method of adoption on the Company’s consolidated financial statements. The Company adopted the new standard effective January 1, 2018 and will use the modified retrospective approach with a cumulative-effect adjustment to retained earnings in the first quarter of 2018. As the Company is still in the process of completing its assessment of its arrangements, an estimate of the potential impact has not yet been made. The Company will complete its assessment in the first quarter of 2018. However, the Company expects the adoption of ASU 2014-09 will have a significant change on the financial statement disclosures.

In February 2016, the FASB issued ASU No. 2016-02, Leases (“ASU 2016-02”), which applies to all leases and will require lessees to record most leases on the balance sheet, but recognize expense in a manner similar to the current standard. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018 and interim periods within those years. Entities are required to use a modified retrospective approach of adoption for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements. Full retrospective application is prohibited. The Company is evaluating the potential impact that the adoption of ASU 2016-02 will have on the Company’s consolidated financial statements.

In March 2016, the FASB, issued ASU No. 2016-09, Compensation - Stock Compensation (“ASU 2016-09”). ASU 2016-09 simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification in the statement of cash flows. Under this guidance, a company recognizes all excess tax benefits and tax deficiencies as income tax expense or benefit in the income statement as the awards vest or are settled. ASU 2016-09 is effective for public companies for annual reporting periods beginning after December 15, 2016, including interim periods within those annual reporting periods. Upon adoption of this standard on January 1, 2017, the Company recognized previously unrecognized excess tax benefits using the modified retrospective transition method, which resulted in a cumulative-effect increase of $179,000 to deferred tax assets which is offset by a corresponding decrease to the valuation allowance. The implementation of ASU 2016-09 did not have a material impact on stock-based compensation expense. As part of the adoption of ASU 2016-09, the Company elected to record forfeitures as they occur.

In October 2016, the FASB issued ASU No. 2016-18, Restricted Cash (“ASU 2016-18”), which requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and restricted cash or restricted cash equivalents. Therefore, amounts described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 was effective for fiscal years beginning after December 15, 2017, and interim periods within those years. Early adoption was permitted. The guidance is effective on a retrospective basis. The Company elected to early adopt this guidance as of October 1, 2017. The Company reclassified restricted cash in the statements of cash flows to be included in the cash and cash equivalents balance. The reclassification was not material to the periods presented. See the “Cash, cash equivalents, and restricted cash” section of this note for additional information.

In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (“ASU 2017-01”), which clarified the definition of a business and provides a screen to determine when an integrated set of assets and activities is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired, or disposed of, is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. This new standard was effective for fiscal years beginning after December 15, 2017, and interim periods within those years. Early adoption was permitted. The Company adopted this new standard as of January 1, 2017, with prospective application to any business development transactions.

In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (“ASU 2017-09”), which provided guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. The new guidance requires modification accounting if the vesting condition, fair value or award classification is not the same both before and after a change to the terms and conditions of the award. This new standard was effective for fiscal years beginning after December 15, 2017, and interim periods within those years. Early adoption was permitted. The Company does not anticipate a material impact to its consolidated financial statements as a result of the adoption of this standard.

Cash Equivalents & Marketable Securities

12 Months Ended

Dec. 31, 2017

Cash Equivalents & Marketable Securities

Cash Equivalents & Marketable Securities

3. Cash Equivalents & Marketable Securities

Cash equivalents and marketable securities consisted of the following at December 31, 2017 (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Cost		Gains		Losses		Value
Cash equivalents:
Money market funds	$	134,635	$	—	$	—	$	134,635
U.S. Treasuries		11,995		—		—		11,995
Marketable securities:
U.S. Treasuries		123,606		—		(47)		123,559
Government agency securities		58,979		—		(29)		58,950
Total cash equivalents and marketable securities	$	329,215	$	—	$	(76)	$	329,139

Cash equivalents and marketable securities consisted of the following at December 31, 2016 (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
December 31, 2016	Cost		Gains		Losses		Value
Cash equivalents:
Money market funds	$	185,323	$	—	$	—	$	185,323
Total cash equivalents and marketable securities	$	185,323	$	—	$	—	$	185,323

At December 31, 2017, the Company held 25 securities that were in an unrealized loss position. The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months at December 31, 2017 was $174.0 million, and there were no securities held by the Company in an unrealized loss position for more than 12 months. As of December 31, 2017, the Company did not intend to sell, and would not be more likely than not required to sell, the securities in an unrealized loss position before recovery of their amortized cost bases. Furthermore, the Company has determined that there was no material change in the credit risk of these securities. As a result, the Company determined it did not hold any securities with any other-than-temporary impairment as of December 31, 2017.

There were no realized gains or losses on available-for-sale securities during the year ended December 31, 2017 and 2016.

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Measurements

Fair Value Measurements

4. Fair Value Measurements

Assets measured at fair value on a recurring basis as of December 31, 2017 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	December 31,		Identical Assets		Inputs		Inputs
Financial Assets	2017		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents
Money market funds	$	134,635	$	134,635	$	—	$	—
U.S. Treasuries		11,995		11,995		—		—
Marketable securities:
U.S. Treasuries		123,559		123,559		—		—
Government agency securities		58,950		58,950		—		—
Money market funds, included in restricted cash		1,619		1,619		—		—
Total financial assets	$	330,758	$	330,758	$	—	$	—

Assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	December 31,		Identical Assets		Inputs		Inputs
Financial Assets	2016		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents	$	185,323	$	185,323	$	—	$	—
Money market funds, included in other current assets		1,619		1,619		—		—
Total financial assets	$	186,942	$	186,942	$	—	$	—

There were no transfers between fair value measurement levels during the years ended December 31, 2017 or 2016.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Prepaid Expenses and Other Current Assets

12 Months Ended

Dec. 31, 2017

Prepaid Expenses and Other Current Assets

Prepaid Expenses and Other Current Assets

5. Prepaid Expenses and Other Current Assets

Prepaid expense and other current assets consisted of the following (in thousands):


	As of
	December 31,
	2017		2016
Prepaid expenses	$	1,864	$	1,662
Other		517		110
Total	$	2,381	$	1,772

Property and Equipment, net

12 Months Ended

Dec. 31, 2017

Property and Equipment, net

Property and Equipment, net

6. Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	As of
	December 31,
	2017		2016
Building	$	35,167	$	35,941
Laboratory equipment		7,415		5,130
Computer equipment		550		392
Leasehold improvements		177		200
Furniture and office equipment		96		170
Software		95		101
Total property and equipment		43,500		41,934
Less: accumulated depreciation		(4,058)		(1,556)
Property and equipment, net	$	39,442	$	40,378

The Company recorded $2.7 million, $1.2 million, and $0.5 million in depreciation expense during the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2017

Accrued Expenses

Accrued Expenses

7. Accrued Expenses

Accrued expenses consisted of the following (in thousands):


	As of
	December 31,
	2017		2016
Employee related expenses	$	3,708	$	2,480
Intellectual property and patent related fees		2,370		13,251
Process and platform development expenses		2,301		443
Success payment expenses		2,000		—
Professional service expenses		487		729
Other expenses		183		536
Total	$	11,049	$	17,439

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Commitments and Contingencies

8. Commitments and Contingencies

Hurley Street Lease

In February 2016, the Company entered into a lease agreement for 59,783 square feet of office and laboratory space located on Hurley Street in Cambridge, Massachusetts. The term of the lease began on October 1, 2016. In connection with the lease and as a security deposit, the Company deposited with the landlord a letter of credit in the amount of approximately $1.6 million. Subject to the terms of the lease and certain reduction requirements specified therein, the $1.6 million security deposit may decrease over time. The letter of credit, which is collateralized by the Company with cash held in a money market account, is recorded in restricted cash and other non-current assets in the accompanying consolidated financial statement as of December 31, 2017 and December 31, 2016.

In connection with this lease, the landlord provided a tenant improvement allowance for costs associated with the design, engineering, and construction of tenant improvements for the leased facility. For accounting purposes, the Company was deemed the owner of the building during the construction period due to the fact that the Company was involved in the construction project, including having responsibilities for cost overruns for planned tenant improvements that did not qualify as “normal tenant improvements” under the lease accounting guidance. Throughout the construction period, the Company recorded the project construction costs incurred as an asset, along with a corresponding facility lease obligation, on its balance sheet for the total amount of the project costs incurred whether funded by the Company or the landlord.

Construction was completed in October 2016, and the Company considered the requirements for sale-leaseback accounting treatment, which included an evaluation of whether all risks of ownership had transferred back to the landlord, as evidenced by a lack of continuing involvement in the leased property. The Company determined that the arrangement did not qualify for sale-leaseback accounting treatment, the building asset would remain on the Company’s balance sheet at its historical cost, and such asset would be depreciated over its estimated useful life of 30 years.

The Company bifurcates its future lease payments pursuant to the Hurley Street lease into (i) a portion that is allocated to the building and (ii) a portion that is allocated to the land on which the building is located, which is recorded as rental expense. Although the Company did not begin making lease payments pursuant to the Hurley Street lease until November 2016, the portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced upon execution of the Hurley Street lease in February 2016.

The lease will continue until October 2023. The Company has the option to extend the lease for an additional five year term at market-based rates. The Company began using this space as its headquarters in October 2016 and rental payments for this property began in November 2016. The base rent is subject to increases over the term of the lease. The non-cancelable minimum annual lease payments, excluding the Company’s share of the facility operating expenses and other costs that are reimbursable to the landlord under the lease, consist of the following (in thousands):


Year ended December 31,	11 Hurley Street Lease
2018		4,055
2019		4,155
2020		4,257
2021		4,362
2022		4,470
2023 and thereafter		3,801
Total minimum lease payments	$	25,100

Rent expense of approximately $1.2 million, $2.5 million, and $1.0 million was incurred during the years ended December 31, 2017, 2016 and 2015, respectively.

In February 2017, the Company subleased approximately 10,000 square feet of the Hurley Street premises pursuant to a sublease (the “Sublease”). Under the terms of the Sublease, the total minimum rental revenue to be received in the future is $0.4 million as of December 31, 2017. The Sublease commenced in February 2017 and will expire on the eighteen month anniversary thereof, unless it is extended for an additional eighteen month term by the subtenant. If the subtenant elects to extend the term of the lease, the base rent is subject to a minimal increase for the subsequent eighteen month period, which is recorded as other income in the consolidated statements of operations

Licensor Expense Reimbursement

The Company is obligated to reimburse The Broad Institute, Inc. (“Broad”) and the President and Fellows of Harvard College (“Harvard”) for expenses incurred by each of them associated with the prosecution and maintenance of the patent rights that the Company licenses from them pursuant to the license agreement by and among the Company, Broad and Harvard, including the interference and opposition proceedings involving patents licensed to the Company under the license agreement. As such, the Company anticipates that it has a substantial commitment in connection with these proceedings until such time as these proceedings have been resolved, but the amount of such commitment is not determinable. The Company incurred an aggregate of $18.2 million, $23.1 million, and $9.4 million in expense during the years ended December 31, 2017, 2016 and 2015, respectively, for such reimbursement.

Success Payments

In 2016, the Company entered into patent license agreements with each of The General Hospital Corporation, d/b/a Massachusetts General Hospital (“MGH”), and Broad (collectively, the “2016 License Agreements”). Pursuant to the terms of the 2016 License Agreements, the Company is required to make certain success payments to MGH, Broad and Wageningen University (“Wageningen” and such payments, collectively, the “Success Payments”), payable in cash or, at the Company’s election common stock in the case of MGH or, in the case of Broad and Wageningen, promissory notes payable in cash or, at the Company’s election subject to certain conditions, common stock of the Company. The Success Payments are payable, if and when, the Company’s market capitalization reaches specified thresholds for a specific period of time or upon a sale of the Company for consideration in excess of those thresholds, as discussed more fully in Note 9 (collectively, the “Payment Conditions”).

The Success Payments were accounted for under the provisions of FASB ASC, Topic 505-50, Equity-Based Payments to Non-Employees. The Company has the right to terminate any of the 2016 License Agreements at will upon written notice. Absent any of the Payment Conditions being achieved prior to termination, the Company would not be obligated to pay any Success Payments. As such, the Company will recognize the expense and liability associated with each Success Payment upon achievement of the associated Payment Conditions, if ever. The expense is recorded as a research and development expense in the statements of operations.

The Company triggered the first Success Payment under one of the 2016 License Agreements during the first quarter of 2017 when the Company’s market capitalization reached $750 million. On March 28, 2017, the Company issued promissory notes for an aggregate principal amount of $5.0 million to Broad and Wageningen and the Company settled such notes in August 2017.

The Company triggered the second Success Payment under one of the 2016 License Agreements during the fourth quarter of 2017 when the Company’s market capitalization reached $1.0 billion. On December 6, 2017, the Company issued promissory notes for an aggregate principal amount of $7.5 million to Broad and the Company settled such notes in January 2018.

The Company triggered the first Success Payment under the MGH license agreement during the fourth quarter of 2017 when the Company’s market capitalization reached $1.0 billion (the “First MGH Success Payment”). The Company accrued $2.0 million relating to the First MGH Success Payment owed to MGH which is included in accrued expense on the consolidated balance sheet for the year ended December 31, 2017. The Company settled this liability in shares of common stock in January 2018.

The Broad and Wageningen Success Payments are discussed more fully within the Notes Payable section below.

Notes Payable

In December 2016, in connection with the Company’s entry into the Cpf1 license agreement with Broad (the “Cpf1 License Agreement”), one of the 2016 License Agreements, it issued promissory notes in an aggregate principal amount of $10.0 million to Broad and Wageningen (the “Initial Notes”). Outstanding principal and accrued interest on the Initial Notes were due and payable on the earlier of December 2017 or a specified period of time following a Company sale or change of control event. The Initial Notes accrued interest at a rate of 4.8% per annum. The Company fully settled the outstanding principal and accrued interest on the Initial Notes by paying $0.2 million in cash to Wageningen in August 2017 and issuing 108,104 shares and 371,166 shares of common stock to Broad in August 2017 and September 2017, respectively, in connection with such settlement.

In March 2017, a $5.0 million Success Payment under the Cpf1 License Agreement became due upon the market capitalization of the Company’s common stock reaching $750 million. The Company issued a promissory note to each of Broad and Wageningen in an aggregate original principal amount of $5.0 million (collectively, the “ March Success Payment Notes”). Outstanding principal and accrued interest on the March Success Payment Notes were due and payable in August 2017. The March Success Payment Notes were subject to the same interest and terms as the Initial Notes, other than the maturity date. The Company settled the outstanding principal and accrued interest on the March Success Payment Notes in August 2017 by paying $0.4 million in cash to Wageningen and issuing 271,347 shares of common stock to Broad in August 2017 in connection with the settlement of the March Success Payment Notes. In September 2017, Wageningen designated Broad as the recipient of any future promissory notes that are owed to Wageningen pursuant to the Cpf1 License Agreement.

In December 2017, $7.5 million in Success Payments under the Cpf1 License Agreement and the Cas9-II license agreement with the Broad (the “Cas9-II License Agreement”), one of the 2016 License Agreements, became due upon the Company’s market capitalization reaching $1.0 billion. The Company issued promissory notes to Broad in an aggregate original principal amount of $7.5 million (collectively, the “December Success Payment Notes”). Outstanding principal and accrued interest on the December Success Payment Notes are due and payable in May 2018. The December Success Payment Notes are subject to the same interest and terms as the Initial Notes, other than the maturity date. The December Success Payment Notes were fully settled in shares of common stock in January 2018 (see Note 18).

The Company believes that the carrying value of the December Success Payments Notes approximates their fair value based on Level 3 inputs including a quoted rate.

Litigation

The Company is not a party to any litigation and did not have contingency reserves established for any litigation liabilities as of December 31, 2017 and 2016.

Significant Agreements	12 Months Ended
Significant Agreements	Dec. 31, 2017
Significant Agreements
Significant Agreements	9. Significant Agreements Juno Therapeutics Collaboration Agreement Summary of Agreement In May 2015, the Company entered into a collaboration and license agreement (the “Collaboration Agreement”) with Juno Therapeutics. The collaboration is focused on the research and development of engineered T cells with chimeric antigen receptors (“CARs”) and T cell receptors (“TCRs”) that have been genetically modified to recognize and kill other cells. The parties will pursue the research and development of CAR and TCR engineered T cell products utilizing the Company’s genome editing technologies with Juno Therapeutics’ CAR and TCR technologies across three research areas. The collaborative program of research to be undertaken by the parties pursuant to the Collaboration Agreement will be conducted in accordance with a mutually agreed upon research plan which outlines each party’s research and development responsibilities across the three research areas. The Company’s research and development responsibilities under the research plan are related to generating genome editing reagents that modify gene targets selected by Juno Therapeutics. Juno Therapeutics is responsible for evaluating and selecting for further research and development CAR and TCR engineered T cell products modified with the Company’s genome editing reagents. Except with respect to the Company’s obligations under the mutually agreed upon research plan, Juno Therapeutics has sole responsibility, at its own cost, for the worldwide research, development, manufacturing and commercialization of products within each of the three research areas for the diagnosis, treatment or prevention of any cancer in humans through the use of engineered T cells, excluding the diagnosis, treatment or prevention of medullary cystic kidney disease 1 (the “Exclusive Field”). The initial term of the research program commenced on May 26, 2015 and continues for five years ending on May 26, 2020 (the “Initial Research Program Term”). Juno Therapeutics may extend the Initial Research Program Term for up to two additional one year periods upon the payment of extension fees for each one year extension period, assuming the Company has agreed to the extension request(s) (together, the initial term and any extension period(s) are referred to as the “Research Program Term”). Under the terms of the Collaboration Agreement, the Company granted to Juno Therapeutics during the Research Program Term a nonexclusive, worldwide, royalty‑free, sublicensable (subject to certain conditions) license under certain of the intellectual property controlled by the Company solely for the purpose of conducting the following activities required under the specified research under the Collaboration Agreement: (i) conduct activities assigned to Juno Therapeutics under the research plan, (ii) conduct activities assigned to the Company under the research plan that the Company fails or refuses to conduct in a timely manner, (iii) use certain genome editing reagents generated under the research program to research, evaluate and conduct preclinical testing and development of certain engineered T cells and (iv) evaluate the data developed in the conduct of activities under the research plan (the “Research License”). Additionally, as it relates to two of the three research areas, the Company granted to Juno Therapeutics an exclusive, milestone and royalty‑bearing, sublicensable license under certain of the intellectual property controlled by the Company to research, develop, make and have made, use, offer for sale, sell, import and export selected CAR and TCR engineered T cell products in the Exclusive Field on a worldwide basis, specifically as it relates to certain targets selected by Juno Therapeutics pursuant to the research program. Furthermore, as it relates to the same two research areas, the Company granted to Juno Therapeutics a non‑exclusive, milestone and royalty‑bearing, sub licensable license under certain of the intellectual property controlled by the Company to use genome editing reagents generated under the research program that are used in the creation of certain CAR or TCR engineered T cell products on which Juno Therapeutics has filed an investigational new drug (“IND”) application in the Exclusive Field for the treatment or prevention of a cancer in humans to research, develop, make and have made, use, offer for sale, sell, import and export those CAR or TCR engineered T cell products in all fields outside of the Exclusive Field (the “Non‑Exclusive Field”) on a worldwide basis, specifically as it relates to certain targets selected by Juno Therapeutics pursuant to the research program (together, the license in the Exclusive Field and the license in the Non‑Exclusive Field are referred to as the “Development and Commercialization License” for each particular research area). Lastly, as it relates to the third research area, the Company granted to Juno Therapeutics a milestone and royalty‑bearing, sublicensable license under certain of the intellectual property controlled by the Company to use the genome editing reagents generated under the research program that are associated with certain CAR or TCR engineered T cell products to research, develop, make and have made, use, offer for sale, sell, import or export those CAR or TCR engineered T cell products in the Exclusive Field on a worldwide basis, specifically as it relates to certain products selected by Juno Therapeutics pursuant to the research program. The license associated with the third research area is exclusive as it relates to CAR or TCR engineered T cell products directed to certain targets as selected by Juno Therapeutics, but is otherwise non‑exclusive (referred to as the “Development and Commercialization License” for the third research area). The Collaboration Agreement will be managed on an overall basis by a project leader from each of the Company and Juno Therapeutics. The project leaders will serve as the contact point between the parties with respect to the research program and will be primarily responsible for facilitating the flow of information, interaction, and collaboration between the parties. In addition, the activities under the Collaboration Agreement during the Research Program Term will be governed by a joint research committee (“JRC”) formed by an equal number of representatives from the Company and Juno Therapeutics. The JRC will oversee, review and recommend direction of the research program. Among other responsibilities, the JRC will monitor and report research progress and ensure open and frequent exchange between the parties regarding research program activities. Under the terms of the Collaboration Agreement, the Company received a $25.0 million up‑front, non‑refundable, non‑creditable cash payment. In addition, Juno Therapeutics is obligated to pay to the Company an aggregate of up to $22.0 million in research and development funding over the Initial Research Program Term across the three research areas consisting primarily of funding for up to a specified maximum number of full time equivalents personnel each year over the Initial Research Program Term across three research areas. Under the terms of the Collaboration Agreement, there is no incremental compensation due to the Company with respect to the Development and Commercialization License granted to Juno Therapeutics associated with the first target or product, as applicable, designated by Juno Therapeutics within each of the three research areas. However, for two of the three research areas, Juno Therapeutics has the option to purchase up to three additional Development and Commercialization Licenses associated with other gene targets for an additional fee of approximately $2.5 million per target. In addition, Juno Therapeutics would be required to make certain milestone payments to the Company upon the achievement of specified development, regulatory and commercial events. More specifically, for the first product to achieve the associated event in each of the three research areas, the Company is eligible to receive up to a $77.5 million in development milestone payments and up to $80.0 million in regulatory milestone payments. In addition, the Company is eligible to receive additional development and regulatory milestone payments for subsequent products developed within each of the three research areas. Moreover, the Company is eligible for up to $75.0 million in commercial milestone payments associated with aggregate sales of all products within each of the three research areas. Development milestone payments are triggered upon the achievement of certain specified development criteria or upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon approval to market a product candidate by the United States Food and Drug Administration (“FDA”) or other global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by the licensee. In addition, to the extent any of the product candidates covered by the licenses conveyed to Juno Therapeutics are commercialized, the Company would be entitled to receive tiered royalty payments of low double digits based on a percentage of net sales. Royalty payments are subject to certain reductions, including for any royalty payments required to be made by Juno Therapeutics related to a third‑party’s intellectual property rights, subject to an aggregate minimum floor. Royalties are due on a licensed product‑by‑licensed product and country‑by‑country basis from the date of the first commercial sale of each product in a country until the later of: (i) the tenth anniversary of the first commercial sale of such licensed product in such country and (ii) the expiration date in such country of the last to expire valid claim within the licensed intellectual property covering the manufacture, use or sale of such licensed product in such country. The Company achieved $2.5 million development milestones under the Collaboration Agreement resulting from technical progress in a research program in each of May 2016 (the “First Milestone”) and July 2017 (the “Second Milestone”). Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, no additional milestone or royalty payments may ever be received from Juno Therapeutics. As of December 31, 2017, the next potential milestone payment that the Company may be entitled to receive under the Collaboration Agreement is a substantive milestone payment of $2.5 million for the achievement of certain development criteria. The Company would recognize the milestone payment as revenue upon achievement. There are no cancellation, termination or refund provisions in the Collaboration Agreement that contain material financial consequences to the Company. Unless earlier terminated, the Collaboration Agreement will continue in full force and effect, on a product‑by‑product and country‑by‑country basis until the date no further payments are due to the Company from Juno Therapeutics. Either party may terminate the Collaboration Agreement if the other party has materially breached or defaulted in the performance of any of its material obligations and such breach or default continues after the specified cure period. Either party may terminate the Collaboration Agreement in the event of the commencement of any proceeding in or for bankruptcy, insolvency, dissolution or winding up by or against the other party that is not dismissed or otherwise disposed of within a specified time period. Juno Therapeutics may terminate the Collaboration Agreement for convenience upon not less than six months prior written notice to the Company. The Company may terminate the Collaboration Agreement in the event that Juno Therapeutics brings, assumes, or participates in, or knowingly, willfully or recklessly assists in bringing a dispute or challenge against the Company related to its intellectual property. Termination of the Collaboration Agreement for any reason does not release either party from any liability which, at the time of such termination, has already accrued to the other party or which is attributable to a period prior to such termination nor preclude either party from pursuing any rights and remedies it may have under the agreement or at law or in equity with respect to any breach of the Collaboration Agreement. If Juno Therapeutics terminates the Collaboration Agreement as a result of the Company’s uncured material breach or default, then: (i) the licenses and rights conveyed to Juno Therapeutics will continue as set forth in the agreement, (ii) Juno Therapeutics’ obligations related to milestones and royalties will continue as set forth in the agreement and (iii) Juno Therapeutics’ rights to prosecute, maintain and enforce certain intellectual property rights will continue as set forth in the agreement. If Juno Therapeutics terminates the Collaboration Agreement for convenience or if the Company terminates the Collaboration Agreement as a result of Juno Therapeutics’ uncured material breach or default, then the licenses conveyed to Juno Therapeutics will terminate. Accounting Analysis The Company evaluated the Collaboration Agreement in accordance with the provisions of ASC, Topic 605-25, Revenue Recognition—Multiple Element Arrangements (“ASC 605-25”). The Company’s arrangement with Juno Therapeutics contains the following deliverables: (i) research and development services during the Initial Research Program Term (the “R&D Services Deliverable”), (ii) the Research License, (iii) the Development and Commercialization Licenses related to each of the three research areas (each, the “Development and Commercialization License Deliverable” for the respective research area), (iv) significant and incremental discount related to the option to purchase up to three additional Development and Commercialization Licenses for two of the research areas (each, the “Discount Deliverable” for the associated option) and (v) JRC services during the Initial Research Program Term (the “JRC Deliverable”). The Company has determined that the options to purchase additional development and commercialization licenses within two of the research program areas related to other gene targets are substantive options. Juno Therapeutics is not contractually obligated to exercise the options. Moreover, as a result of the uncertain outcome of the discovery, research and development activities, there is significant uncertainty as to whether Juno Therapeutics will decide to exercise its option for any additional gene targets within either of the two applicable research areas. Consequently, the Company is at risk with regard to whether Juno Therapeutics will exercise the options. However, the Company has determined that the options to purchase additional development and commercialization licenses with respect to other gene targets within the two applicable research program areas are priced at a significant and incremental discount. As a result, the Company has concluded that the discounts to purchase development and commercialization licenses for up to three additional gene targets within both of the research areas represent separate elements in the arrangement at inception. Accordingly, the deliverables identified at inception of the arrangement include six separate deliverables related to the significant and incremental discount inherent in the pricing of the option to purchase up to three additional development and commercialization licenses for two of the research areas included within the research program. The Company has concluded that the Research License deliverable does not qualify for separation from the R&D Services Deliverable. As it relates to the assessment of standalone value, the Company has determined that Juno Therapeutics cannot fully exploit the value of the Research License deliverable without receipt of the R&D Services Deliverable. This is primarily due to the fact that Juno Therapeutics must rely upon the Company to provide the research and development services included in the research plan because the services incorporate technology that is proprietary to the Company. The services to be provided by the Company involve unique skills and specialized expertise, particularly as it relates to genome editing technology that is not available in the marketplace. Accordingly, Juno Therapeutics must obtain the research and development services from the Company which significantly limits the ability for Juno Therapeutics to utilize the Research License for its intended purpose on a standalone basis. Therefore, the Research License deliverable does not have standalone value from the R&D Services Deliverable. As a result, the Research License deliverable and the R&D Services Deliverable have been combined as a single unit of accounting (the “R&D Services Unit of Accounting”). Conversely, the Company has concluded that each of the other deliverables identified at the inception of the arrangement has standalone value from each of the other elements based on their nature. Factors considered in this determination included, among other things, the capabilities of the collaboration partner, whether any other vendor sells the item separately, whether the value of the deliverable is dependent on the other elements in the arrangement, whether there are other vendors that can provide the items and if the customer could use the item for its intended purpose without the other deliverables in the arrangement. Additionally, the Collaboration Agreement does not include a general right of return. Accordingly, each of the other deliverables included in the Juno Therapeutics arrangement qualifies as a separate unit of accounting. Therefore, the Company has identified eleven units of accounting in connection with its obligations under the collaboration arrangement with Juno Therapeutics as follows: (i) the R&D Services Unit of Accounting, (ii) three units of accounting related to the Development and Commercialization Licenses for each of the three research areas, (iii) six units of accounting related to each of the Discount Deliverables, and (iv) the JRC Deliverable. The Company has determined that neither vendor specific objective evidence of selling price nor third-party evidence of selling price is available for any of the units of accounting identified at inception of the arrangement with Juno Therapeutics. Accordingly, the selling price of each unit of accounting was determined based on the Company’s BESP. The Company developed the BESP for all of the units of accounting included in the Collaboration Agreement with the objective of determining the price at which it would sell such an item if it were to be sold regularly on a standalone basis. The Company developed the BESP for the R&D Services Unit of Accounting and the JRC Deliverable primarily based on the nature of the services to be performed and estimates of the associated effort and cost of the services, adjusted for a reasonable profit margin that would be expected to be realized under similar contracts. The Company developed the BESP for each of the Development and Commercialization License units of accounting based on the probability-weighted present value of expected future cash flows associated with each license related to each specific research area. In developing such estimate, the Company also considered applicable market conditions and relevant entity-specific factors, including those factors contemplated in negotiating the agreement, probability of success and the time needed to commercialize a product candidate pursuant to the associated license. The Company developed the BESP for each of the Discount Deliverables based on the estimated value of the associated in-the-money options. In developing such estimate, the Company considered the period to exercise the option, an appropriate discount rate and the likelihood that a market participant who was entitled to the discount would exercise the option. Allocable arrangement consideration at inception is comprised of: (i) the up‑front payment of $25.0 million, (ii) the research support of $20.0 million and (iii) payments related to specialized materials costs of $2.0 million. The research support of $20.0 million and payments related to specialized materials costs of $2.0 million represent contingent revenue features because the Company’s retention of the associated arrangement consideration is dependent upon its future performance of research support services and development of specialized materials. The aggregate allocable arrangement consideration of $47.0 million was allocated among the separate units of accounting using the relative selling price method as follows: (i) R&D Services Unit of Accounting: $16.7 million, (ii) Development and Commercialization License for the first research area: $9.3 million, (iii) Development and Commercialization License for the second research area: $15.4 million, (iv) Development and Commercialization License for the third research area: $0.2 million, (v) the first Discount Deliverable for the first research area: $0.7 million, (vi) the second Discount Deliverable for the first research area: $0.4 million, (vii) the third Discount Deliverable for the first research area: $0.2 million, (viii) the first Discount Deliverable for the second research area: $2.0 million, (ix) the second Discount Deliverable for the second research area: $1.3 million, and (x) the third Discount Deliverable for the second research area: $0.8 million. No amounts were allocated to the JRC Deliverable because the associated BESP was determined to be de minimis. The amounts allocated to each of the development and commercialization licenses are based on the respective BESP calculations, which reflect the level of risk and expected probability of success inherent in the nature of the associated research area. The Company will recognize revenue related to amounts allocated to the R&D Services Unit of Accounting as the underlying services are performed. The Company will recognize revenue related to amounts allocated to each of the Development and Commercialization Licenses upon delivery of the associated license, assuming the research services are substantially complete at the time the license is delivered. The rights to be conveyed to Juno Therapeutics pursuant to each of the Development and Commercialization Licenses extend exclusively to an individual target or product, as applicable; therefore, delivery is deemed to occur upon the designation by Juno Therapeutics of the specific target or product, as applicable, whereupon the license becomes effective. The Company will recognize revenue related to amounts allocated to each of the Discount Deliverables upon the earlier of exercise of the associated option or upon lapsing of the underlying right, if the respective option expires unexercised. The Company has evaluated all of the milestones that may be received in connection with the Juno Therapeutics arrangement. In evaluating if a milestone is substantive, the Company assesses whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance, and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. All development and regulatory milestones are considered substantive on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the milestone as well as the level of effort and investment required. Accordingly, such amounts will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All commercial milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met. During the year ended December 31, 2017, 2016 and 2015, the Company recognized revenue totaling approximately $4.9 million, inclusive of the Second Milestone payment, $5.7 million, inclusive of the First Milestone payment, and $1.6 million, respectively, under the collaboration with Juno Therapeutics. The revenue is classified as collaboration and other research and development revenue in the accompanying consolidated statement of operations. As of December 31, 2017 and 2016, there was approximately $26.4 million and $26.0 million of deferred revenue related to the Company’s collaboration with Juno Therapeutics, respectively, all of which is classified as long-term in the accompanying consolidated balance sheet. In addition, as of December 31, 2017, the Company has recorded accounts receivable of $0.5 million related to reimbursable research and development costs under the Collaboration Agreement for activities performed during the fourth quarter of 2017. There was no receivable balance as of December 31, 2016. During the years ended December 31, 2017 and 2016, the Company paid $0.5 million and $0.5 million in sublicense fees that were owed to certain of the Company’s licensors in connection with the Second Milestone and First Milestone, respectively, which the Company recorded as research and development expenses during such periods. Allergan Pharmaceuticals Strategic Alliance and Option Agreement Summary of Agreement In March 2017, the Company entered into a Strategic Alliance and Option Agreement with Allergan to discover, develop, and commercialize new gene editing medicines for a range of ocular disorders (the “Allergan Agreement”). Over a seven-year research term, Allergan will have an exclusive option to exclusively license from the Company up to five collaboration development programs for the treatment of ocular disorders (each a “CDP”), including the Company’s Leber’s Congenital Amaurosis type 10 program (the “LCA10 Program”). Under the Allergan Agreement, the Company will use commercially reasonable efforts to develop at least five CDPs and deliver preclinical results and data meeting specified criteria with respect to each CDP (each, an “Option Package” and such criteria, the “Option Package Criteria”) to Allergan. The list of proposed targets that may be subject to a CDP may be amended from time to time by mutual agreement of the Company and Allergan. The Company is responsible for the preparation and delivery of a written development plan for each particular CDP setting forth the discovery and research activities to be conducted which is subject to the approval of the alliance steering committee that was formed under the Allergan Agreement, comprised of three members from each of the Company and Allergan (the “Steering Committee”). The Company will maintain primary responsibility for the development efforts under each CDP. The Company is responsible for all research and development costs prior to the achievement of the Option Package Criteria. Upon achievement of the Option Package Criteria, as determined by the Steering Committee, Allergan will have the ability, for a defined period of time (“Initial Option Period”) to exercise an option (each, an “Option”) to obtain a world-wide right and license to the Company’s background intellectual property and the Company’s interest in the CDP intellectual property to develop, commercialize, make, have made, use, offer for sale, sell, and import any gene editing therapy product that results from such CDP during the term of the Allergan Agreement (a “Licensed Product”) in any category of human diseases and conditions other than the diagnosis, treatment or prevention of any cancer in humans through the use of engineered T-cells and subject to specified other limitations. Allergan has the option to extend the Initial Option Period and require the Company to perform additional research and development services, subject to the payment of additional consideration. After exercise of an Option with respect to a CDP, with the exception of any CDP’s where the Company has exercised its profit-sharing option, Allergan will be responsible for all development, manufacturing, and commercialization activities in connection with licensed products arising from such CDP, other than with respect to the LCA10 Program, if LCA10 is designated as a CDP, for which the Company has retained the right to develop that program through the acceptance for filing of the first IND with respect to the LCA10 Program. Upon achievement of IND approval for LCA10, unless the Company has exercised its profit sharing option on LCA10, Allergan will be responsible for all development, manufacturing, and commercialization activities. The initial term of the Allergan Agreement commenced on March 14, 2017 (the “Effective Date”) and continues for seven years ending on March 14, 2024 (the “Research Term”). If the Company has not delivered an Option Package, which includes the results and data from the CDP, for five CDPs that satisfy the Option Package Criteria, then the Research Term will automatically extend by one-year increments until such obligation is satisfied, up to a maximum of ten years from the Effective Date. The activities under the Allergan Agreement during the Research Term will be governed by the Steering Committee. The Steering Committee will review and monitor the direction of the development plan, evaluate and determine which targets are selected to become CDP, establish the Option Package Criteria for each CDP and evaluate the achievement of such criteria as well as oversee the development and commercialization activities after Allergan has licensed a CDP. Under the terms of the Allergan Agreement, the Company received a $90.0 million up‑front, non‑refundable, non‑creditable cash payment (the “Allergan Upfront”) related to the Company’s research and development costs for Option Packages for at least five CDPs and for reimbursement of the Company’s past out of pocket costs with respect to the prosecution and defense of patents that it owns and in-licenses. Allergan has the option to purchase at least five development and commercialization licenses associated CDP that have satisfied the Option Package Criteria. The option exercise fee during the Initial Option Period is $15.0 million per CDP. If Allergan elects to extend the Initial Option Period, Allergan is required to pay an additional fee of $5.0 million to extend the option, at which point the Company is required to perform additional research services. If Allergan elects to exercise its option to a development and commercialization license after extending the Initial Option Period, Allergan must pay the Company the option exercise fee of $22.5 million, plus specified costs incurred by the Company in connection with the additional development work. Following the exercise by Allergan of an Option with respect to a CDP, Allergan would be required to make certain milestone payments to the Company upon the achievement of specified development, product approval and launch and commercial events, on a CDP by CDP basis. On a CDP by CDP basis, for the first product in the first field to achieve the associated event, the Company is eligible to receive up to an aggregate of $42.0 million for development milestone payments and $75.0 million for product approval and launch milestone payments, in each case, for an indication in the field per CDP. In addition, the Company is eligible to receive additional development and product approval and launch milestone payments for subsequent products developed within two additional fields. The Company is also eligible for up to $90.0 million in sales milestone payments on a CDP by CDP basis, associated with aggregate worldwide sales. Certain product approval milestones are subject to certain reductions under specified circumstances, including for payments required to be made by Allergan to obtain certain third party intellectual property rights. In addition, within 45 days of the acceptance by the applicable regulatory authority of the Company’s submission of an IND application with respect to the LCA10 Program, Allergan is required to pay the Company a one-time payment of $25.0 million (the “LCA10 IND Payment”), whether or not Allergan exercises its option under the Allergan Agreement to acquire an exclusive license with respect to the LCA10 Program. As of December 31, 2017, the next potential milestone payment that the Company may be entitled to receive under the Allergan Agreement is a substantive milestone payment of $8.0 million for the achievement of certain clinical criteria. With respect to the LCA10 Program and up to one other CDP of the Company’s choosing, following the exercise by Allergan of its Option to such programs, the Company will have the right to elect to participate in a profit-sharing arrangement with Allergan in the United States, on terms mutually agreed by the Company and Allergan and subject to a right of Allergan to reject such election under certain circumstances, under which the Company and Allergan would share equally in net profits and losses on specific terms to be agreed between the Company and Allergan, in lieu of Allergan paying royalties on net sales of any applicable Licensed Products in the United States, and in such event Allergan’s milestone payment obligations would be reduced, with the Company being eligible to receive development and product approval and launch milestone payments up to a low nine-digit amount in the aggregate and further sales milestone payments up to a high-eight digit amount in the aggregate, subject to reduction under certain circumstances. If the Company elects to participate in a profit-sharing arrangement, the Company is obligated to reimburse Allergan for half of the development costs incurred by Allergan with respect to the applicable CDP, and Allergan will retain control of all development and commercialization activities for the applicable Licensed Products. In addition, to the extent there is any Licensed Product, the Company would be entitled to receive tiered royalty payments of high single digits based on a percentage of net sales of such Licensed Product, subject to certain reductions under specified circumstances, and the Company will remain obligated to pay all license fees, milestone payments, and royalties due to its upstream licensors based on Allergan’s exercise of its license rights with respect to Licensed Products. However, if a Licensed Product is subject to a profit sharing agreement the royalties will only be paid on ex-US net sales. Royalties are due on a Licensed Product‑by‑Licensed Product and country‑by‑country basis from the date of the first commercial sale of each Licensed Product in a country until the later of: (i) the tenth anniversary of the first commercial sale of such Licensed Product in such country (ii) the expiration date in such country of the last to expire valid claim within the licensed intellectual property covering the manufacture, use or sale of such Licensed Product in such country and (iii) the expiration of an exclusive legal right granted by the regulatory authority in such country to market and sell such Licensed Product. Unless earlier terminated, the Allergan Agreement will terminate upon (i) the expiration of the Research Term, if Allergan does not exercise an Option, (ii) on a Licensed Product-by-Licensed Product and country-by-country basis, on the date of the expiration of all payment obligations under the Allergan Agreement with respect to such Licensed Product in such country or (iii) in its entirety upon the expiration of all payment obligations with respect to the last Licensed Product in all countries, unless terminated earlier due to the early termination provisions. Either party may terminate the Allergan Agreement if the other party has materially breached or defaulted in the performance of any of its material obligations and such breach or default continues after the specified cure period. During the Research Term, Allergan will have the right to terminate the Allergan Agreement on a CDP by CDP basis in the event of a change in control of the Company or for all CDPs, provided that Allergan will not have any right to exercise an Option for any CDPs following such termination. After the exercise of an Option, Allergan will have the right, at its sole discretion, to terminate the Allergan Agreement, on a CDP by CDP basis, upon 90 days’ written notice. The Company may terminate the Allergan Agreement in the event that Allergan brings, assumes, or participates in, or knowingly, willfully or recklessly assists in bringing a dispute or challenge against the Company related to its intellectual property. Lastly, Allergan may terminate the Allergan Agreement with respect to a CDP if a safety concern, as specified in the Allergan Agreement, arises. Termination of the Allergan Agreement for any reason will not release either party from any liability which, at the time of such termination, has already accrued to the other party or which is attributable to a period prior to such termination. In addition, termination of the Allergan Agreement will not preclude either party from pursuing any rights and remedies it may have under the agreement or at law or in equity with respect to any breach of the Allergan Agreement. If Allergan terminates the Allergan Agreement as a result of the Company’s uncured material breach or default, then: (i) the licenses and rights conveyed to Allergan will continue as set forth in the agreement for any CDP Allergan has already licensed and (ii) Allergan’s obligations related to milestones and royalties will continue as set forth in the agreement. If the Allergan Agreement is terminated for any other reason, then the options and licenses conveyed to Allergan under the agreement will terminate. Accounting Analysis The Company evaluated the Allergan Agreement in accordance with the provisions of ASC 605-25. The Company’s arrangement with Allergan contains the following deliverables: (i) research and development services during the Research Term (the “Allergan R&D Services Deliverable”), and (ii) ASC services during the Research Term (the “ASC Deliverable”). The Company has determined that the Options with respect to the CDP are substantive options. Allergan is not contractually obligated to exercise the Options and as a result of the uncertain outcome of the discovery, research and development activities as well as the significant option exercise fee payable upon exercise of an Option, there is significant uncertainty as to whether Allergan will decide to exercise its Option for any CDP. Consequently, the Company is at risk with regard to whether Allergan will exercise the Options. In addition, the option exercise fees are not priced at a significant and incremental discount. Accordingly, the substantive options are not considered deliverables at the inception of the arrangement and the associated option exercise payments are not included in allocable arrangement consideration. The Company has also determined that any obligations which are contingent upon the exercise of a substantive option are not considered deliverables at the outset of the arrangement. The Company has concluded that the services being provided as part of the ASC Deliverable does not qualify for separation from the Allergan R&D Services Deliverable. The Steering Committee provides oversight and management of the overall Allergan Agreement, and the members of the Steering Committee from the Company have specialized industry knowledge, particularly as it relates to genome editing technology. The Company has concluded that the Steering Committee is a participatory obligation of the Company and is meant to facilitate the early stage research being performed and coordinate the activities of both the Company and Allergan. Further, the Steering Committee services are critical to the selection of the CDP, the ongoing evaluation of the CDP and the development and evaluation of the Option Package Criteria. Accordingly, the Company’s participation on the Steering Committee is essential to Allergan receiving value from the Allergan R&D Services Deliverable and as such, the ASC Deliverable along with the Allergan R&D Services Deliverable are considered one unit (the “CDP Services Unit”). As the Company concluded that the CDP Services Unit is the sole unit of accounting (the “CDP Services Unit of Accounting”), all of the initial arrangement consideration will be allocated to that unit and no allocation of arrangement consideration is necessary. Allocable arrangement consideration at inception is comprised solely of the up‑front payment of $90.0 million. The Company will recognize revenue related to the CDP Services Unit of Accounting as the underlying services are performed. In addition, as the LCA10 IND Payment is payable upon acceptance of the IND, it is contingent consideration related to the licensed technology. As such, if and when the LCA10 IND Payment is received, the Company will recognize revenue related to the LCA10 IND Payment in conjunction with the CDP Services Unit of Accounting as the underlying services are performed. The Company has evaluated all of the milestones that may be received in connection with the Allergan Agreement. In evaluating if a milestone is substantive, the Company assesses whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance, and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. All development and product approval and launch milestones are considered substantive on the basis of the contingent nature of the milestones, specifically reviewing factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the milestone. Accordingly, such amounts will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All sales milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met. During the year ended December 31, 2017, the Company recognized revenue totaling approximately $8.8 million with respect to the Allergan Agreement. As of December 31, 2017, there was $81.2 million of deferred revenue related to the Allergan Agreement, of which $68.3 million is classified as long-term on the consolidated balance sheet. The Company will recognize revenue on a straight-line basis, as there is no discernible pattern or objective measure of performance of the services, over the estimated performance period. The estimated performance period is from the commencement of providing services related to the CDP Services Unit until the end of the Research Term. During the year ended December 31, 2017, the Company paid $14.1 million in sublicense fees that were owed to certain of the Company’s licensors in connection with the Allergan Upfront, which the Company recorded as research and development expenses during such period. Other Agreements Licensing Agreements The Company is a party to a number of license agreements under which the Company licenses patents, patent applications and other intellectual property from third parties. The Company anticipates entering into these types of license agreements in the future. The Company believes the following agreements are significant to the business: Massachusetts General Hospital Agreements In August 2014, the Company entered into an agreement to license certain patent rights owned or co‑owned by MGH. Consideration for the granting of the license included the payment of an upfront license fee of $0.1 million, the issuance of 66,848 shares of the Company’s common stock, which was based on 0.5% of the Company’s outstanding stock on a fully diluted basis, and the right to receive future issuances of shares of common stock to maintain MGH’s ownership following the third tranche of the Company’s Series A redeemable convertible preferred stock financing (i.e. anti‑dilution protection liability), which was settled in June 2015. MGH is entitled to receive nominal annual license fees and future clinical, regulatory and commercial milestone payments in an aggregate maximum amount of $3.7 million and an aggregate amount of $1.8 million upon the occurrence of certain sales milestones. The Company is also obligated to pay MGH low single digit percentage royalties on net sales of products for the prevention or treatment of human disease, and ranging from low single digit to low double digit percentage royalties on net sales of other products and services made by the Company, its affiliates or its sublicenses. The royalty percentage depends on the product and service, and whether such licensed product or licensed service is covered by a valid claim within the certain patent rights that the Company licenses from MGH. In August 2016, the Company entered into a license agreement with MGH (the “2016 MGH Agreement”) to license certain patent rights owned or co-owned by MGH (the “Additional MGH Patent Rights”). Consideration for granting the license included the payment of an upfront nonrefundable license fee of $0.8 million, which the Company recorded as research and development expense in 2016. Under the 2016 MGH Agreement, MGH is entitled to nominal annual license fees, clinical and regulatory milestone payments totaling less than $1.0 million in the aggregate per licensed product up to four licensed products or processes to achieve the specified clinical and regulatory milestones, and commercial sales milestone payments totaling up to $4.9 million in the aggregate, consisting of milestone payments due upon the first commercial sales for up to four licensed products or processes and milestone payments due upon annual net sales of products or processes meeting specified thresholds. The Company is also obligated to pay MGH royalties of less than 1% on net sales of products and processes for the prevention or treatment of human disease, and royalties of a low single-digit percentage on net sales of products and processes for the prevention or treatment of a non-human animal disease, made by the Company, its affiliates, or its sublicensees. The royalty percentages that the Company is obligated to pay are subject to reduction if at the time of sale the applicable product or process is not covered by a valid claim within the Additional MGH Patent Rights. Under the 2016 MGH Agreement, the Company is obligated to reimburse MGH for all patent costs and future reasonable costs associated with the prosecution, filing, and maintenance of the licensed patents. MGH is also entitled under the 2016 MGH Agreement to receive payments of up to $6.0 million in the event the Company’s market capitalization reaches specified thresholds meeting or exceeding $1.0 billion, on or prior to the expiration or termination of the 2016 MGH Agreement (or if earlier, a Company sale) (“MGH Market Cap Success Payments”) or a Company sale for consideration in excess of those thresholds (“MGH Company Sale Success Payments”). Additional MGH Market Cap Success Payments become payable, and the amount of potential MGH Company Sale Success Payments would increase further, if the Company’s market capitalization reaches additional higher thresholds and the Company has at least one product candidate that is covered by a claim of an Additional MGH Patent Right and that (i) is the subject of a Phase 1 clinical trial of which the Company or an affiliate or sublicensee of the Company is the sponsor, (ii) was the subject of a Phase 1 clinical trial of which the Company or an affiliate or sublicensee of the Company was the sponsor with the Company having determined to conduct a subsequent clinical trial with respect to such product candidate, or (iii) has been approved for sale in either the United States or European Union. MGH Market Cap Success Payments are payable in cash or shares of Company common stock at the Company’s discretion, and MGH Company Sale Success Payments are payable solely in cash. The Company triggered the first Success Payment under the MGH license agreement during the fourth quarter of 2017 when the Company’s market capitalization reached $1.0 billion (see Note 8). The Broad Institute Agreements In October 2014, the Company entered into an agreement (the “Cas9-I License Agreement”) with Broad and Harvard to license certain patent rights owned or co‑owned by, or among, Broad, the Massachusetts Institute of Technology (“MIT”), and Harvard (collectively, the “Institutions”). Consideration for the granting of the license included the payment of an upfront license issuance fee of $0.2 million and the issuance of 561,531 shares of the Company’s common stock. The Institutions are collectively entitled to receive clinical and regulatory milestone payments totaling up to $14.8 million in the aggregate per licensed product approved in the United States, European Union, and Japan for the treatment of a human disease that afflicts at least a specified number of patients in the aggregate in the United States. If the Company undergoes a change of control during the term of the license agreement, the clinical and regulatory milestone payments will be increased by a certain percentage in the mid‑double digits. The Company is also obligated to make additional payments to the Institutions, collectively, of up to an aggregate of $54.0 million upon the occurrence of certain sales milestones per licensed product for the treatment of a human disease that afflicts at least a specified number of patients in the aggregate in the United States. The Institutions are collectively entitled to receive clinical and regulatory milestone payments totaling up to $4.1 million in the aggregate per licensed product approved in the U.S. and at least one jurisdiction outside the U.S. for the treatment of a human disease based on certain criteria. The Company is also obligated to make additional payments to the Institutions, collectively, of up to an aggregate of $36.0 million upon the occurrence of certain sales milestones per licensed product for the treatment of a rare disease meeting certain criteria. The Institutions are entitled to receive from the Company nominal annual license fees and a mid‑single digit percentage royalties on net sales of products for the prevention or treatment of human disease, and ranging from low single digit to high single digit percentage royalties on net sales of other products and services, made by the Company, its affiliates, or its sublicensees. The royalty percentage depends on the product and service, and whether such licensed product or licensed service is covered by a valid claim within the certain patent rights that the Company licenses from the Institutions. In December 2016, the Company entered into the Cpf1 License Agreement with Broad, for specified patent rights (the “Cpf1 Patent Rights”) related primarily to Cpf1 compositions of matter and their use for gene editing. Concurrently with entering into the Cpf1 License Agreement, the Company, Broad, and Harvard amended and restated the Cas9-I License Agreement as described below. Concurrently, the Company and Broad also entered into the Cas9‑II License Agreement for specified patent rights (the “Cas9-II Patent Rights”) related primarily to certain Cas9 compositions of matter and their use for genome editing. The Company paid an upfront fee in aggregate of $16.5 million, which included the Initial Notes, under these agreements which was recorded in research and development expenses during 2016. Cpf1 License Agreement Pursuant to the Cpf1 License Agreement, Broad, on behalf of itself, Harvard, MIT, Wageningen, and the University of Tokyo (“UTokyo” and, together with Broad, Harvard, MIT, and Wageningen, the “Cpf1 Institutions”) granted the Company an exclusive, worldwide, royalty‑bearing, sublicensable license to the Cpf1 Patent Rights, to make, have made, use, have used, sell, offer for sale, have sold, export and import products in the field of the prevention or treatment of human disease using gene therapy, editing of genetic material, or targeting of genetic material, subject to certain limitations and retained rights (collectively, the “Cpf1 Exclusive Field”), as well as a non‑exclusive, worldwide, royalty‑bearing sublicensable license to the Cpf1 Patent Rights for all other purposes, subject to certain limitations and retained rights. The Company is obligated to use commercially reasonable efforts to research, develop, and commercialize products in the Cpf1 Exclusive Field. The Company is also required to achieve certain development milestones within specified time periods for products covered by the Cpf1 Patent Rights, with Broad having the right to terminate the Cpf1 License Agreement if the Company fails to achieve these milestones within the required time periods. Broad and Wageningen are collectively entitled to receive clinical and regulatory milestone payments totaling up to $20.0 million in the aggregate per licensed product approved in the United States, European Union, and Japan for the prevention or treatment of a human disease that afflicts at least a specified number of patients in the aggregate in the United States. The Company is also obligated to make additional payments to Broad and Wageningen, collectively, of up to an aggregate of $54.0 million upon the occurrence of certain sales milestones per licensed product for the prevention or treatment of a human disease that afflicts at least a specified number of patients in the aggregate in the United States. Broad and Wageningen are collectively entitled to receive clinical and regulatory milestone payments totaling up to $6.0 million in the aggregate per licensed product approved in the United States, European Union and Japan for the prevention or treatment of a human disease that afflicts fewer than a specified number of patients in the aggregate in the United States or a specified number of patients per year in the United States (an “Ultra‑Orphan Disease”). The Company is also obligated to make additional payments to Broad and Wageningen, collectively, of up to an aggregate of $36.0 million upon the occurrence of certain sales milestones per licensed product for the prevention or treatment of an Ultra‑Orphan Disease. Broad and Wageningen, collectively, are entitled to receive, on a product‑by‑product and country‑by‑country basis, mid single‑digit percentage royalty on net sales of licensed products for the prevention or treatment of human disease, and royalties on net sales of other licensed products and licensed services, made by the Company, its affiliates, or its sublicensees. The royalty percentage depends on the product and service, and whether such licensed product or licensed service is covered by a valid claim within the Cpf1 Patent Rights. If the Company is legally required to pay royalties to a third party on net sales of the Company’s products because such third party holds patent rights that cover such licensed product, then the Company can credit up to a specified percentage of the amount paid to such third party against the royalties due to Broad and Wageningen in the same period. Such credit may not exceed 50% of the applicable royalties paid by the Company to the applicable third party. The Company’s obligation to pay royalties will expire on a product‑by‑product and country‑by‑country basis upon the later of the expiration of the last to expire valid claim of the Cpf1 Patent Rights that covers each licensed product or service in each country or the tenth anniversary of the date of the first commercial sale of the licensed product or licensed service. If the Company sublicenses any of the Cpf1 Patent Rights to a third party, Broad and Wageningen, collectively, have the right to receive sublicense income, depending on the stage of development of the products or services in question at the time of the sublicense. Under the Cpf1 License Agreement, Broad and Wageningen are also entitled, collectively, to receive success payments in the event the Company’s market capitalization reaches specified thresholds (the “Cpf1 Market Cap Success Payments”) or a Company sale for consideration in excess of those thresholds (the “Cpf1 Company Sale Success Payments” and, collectively with the Cpf1 Market Cap Success Payments, the “Cpf1 Success Payments”). The Cpf1 Success Payments payable to Broad and Wageningen are triggered when the Company’s market capitalization reaches certain amounts ranging from $750 million to $10 billion for a specified period of time, and, collectively, the Cpf1 Success Payments will not exceed, in aggregate, $125.0 million, which maximum amount would be payable only if the Company reaches a market capitalization threshold of $10.0 billion and has at least one product candidate covered by a claim of a patent right licensed to the Company under either the Cpf1 License Agreement or the Cas9‑I License Agreement that is or was the subject of a clinical trial pursuant to development efforts by the Company or any Company affiliate or sublicensee. The Cpf1 Market Cap Success Payments are payable by the Company in cash or in the form of promissory notes on substantially the same terms and conditions as the Initial Notes, as described in Note 8, except that the maturity date of such notes will, subject to certain exceptions, be 150 days following issuance. Following a change in control of the Company, Cpf1 Market Cap Success Payments are required to be made in cash. Cpf1 Company Sale Success Payments are payable solely in cash. The Company triggered the first and second Cpf1 Success Payments during 2017 when the Company’s market capitalization reached $750 million and $1.0 billion, respectively (see Note 8). Unless terminated earlier, the term of the Cpf1 License Agreement will expire on a country‑by‑country basis, upon the expiration of the last to expire valid claim of the Cpf1 Patent Rights in such country. The Company has the right to terminate the Cpf1 License Agreement at will upon four months’ written notice to Broad. Either party may terminate the Cpf1 License Agreement upon a specified period of notice in the event of the other party’s uncured material breach of a material obligation, such notice period varying depending on the nature of the breach. Broad may terminate the Cpf1 License Agreement immediately if the Company challenges the enforceability, validity, or scope of any Cpf1 Patent Right or assist a third party to do so, or in the event of the Company’s bankruptcy or insolvency. Amendment and Restatement of Cas9-I License Agreement In December 2016, the Company amended and restated the Cas9‑I License Agreement (such agreement, as amended, the “Amended and Restated Cas9-I License Agreement”) to exclude additional fields from the scope of the exclusive license previously granted to the Company, to make the exclusive license to three targets become non‑exclusive, subject to the limitation that each of Broad and Harvard would only be permitted to grant a license to only one third party at a time with respect to each such target within the field of the exclusive license, and to revise certain provisions relating to the rights of Harvard and Broad to grant further licenses under specified circumstances to third parties that wish to develop and commercialize products that target a particular gene and that otherwise would fall within the scope of the exclusive license under this agreement, so that Harvard and Broad together would have rights substantially similar to the equivalent rights possessed by Broad under the Cpf1 License Agreement to designate gene targets for which the designating institution, whether alone or together with an affiliate or third party, has an interest in researching and developing products that would otherwise be covered by rights licensed by Harvard and/or Broad to the Company under this agreement, the Cpf1 License Agreement or the Cas9‑II Agreement. In March 2017, the Company and Harvard and Broad further amended the Amended and Restated Cas9-I License Agreement to (i) grant an exclusive license from Broad to the Company with respect to certain patent rights that The Rockefeller University (“Rockefeller”) has or may have rights in and to and for which Rockefeller has, under a certain inter-institutional agreement that Broad and Rockefeller entered into in February 2017, appointed Broad as sole and exclusive agent for the purposes of licensing and (ii) provide to Rockefeller certain rights, including with respect to patent enforcement, indemnification, insurance, confidentiality, reservation of certain rights, and publicity, that are generally consistent with those granted to Broad, Harvard, MIT and the Howard Hughes Medical Institute under the Amended and Restated Cas9-I License Agreement, as amended. Cas9‑II License Agreement Pursuant to the Cas9‑II License Agreement, Broad, on behalf of itself, MIT, Harvard, and the University of Iowa Research Foundation, granted the Company an exclusive, worldwide, royalty bearing sublicensable license to certain of the Cas9‑II Patent Rights as well as a non‑exclusive, worldwide, royalty‑bearing sublicensable license to all of the Cas9‑II Patent Rights, in each case on terms substantially similar to the licenses granted to the Company under the Cpf1 License Agreement except, among other things, for the following commitment amounts. Under the Cas9‑II License Agreement, the Company will pay an upfront license fee in a low seven digit dollar amount and will have to pay an annual license maintenance fee. The Company is obligated to pay clinical and regulatory milestone payments per licensed product approved in the United States, European Union and Japan for the prevention or treatment of a human disease that afflicts at least a specified number of patients in the aggregate in the United States totaling up to $3.7 million in the aggregate, and sales milestone payments for any such licensed product totaling up to $13.5 million in the aggregate. In addition, the Company is obligated to pay clinical and regulatory milestone payments totaling up to $1.1 million in the aggregate per licensed product approved in the United States and the European Union or Japan for the prevention or treatment of a human disease that afflicts fewer than a specified number of patients in the United States, plus sales milestone payments of up to $9.0 million for any such licensed product. Consistent with the Cpf1 License Agreement, the licensors are entitled to royalties on net sales of products for the prevention or treatment of human disease and other products and services made by the Company, its affiliates, or its sublicensees. Royalties due under other license agreements are creditable against these royalties up to a specified amount in the same period. Lastly, Broad is entitled to receive success payments if the Company’s market capitalization reaches specified thresholds ascending from $1.0 billion to $9.0 billion or upon a sale of the Company for consideration in excess of those thresholds. The potential success payments range from a low seven digit dollar amount to a low eight digit dollar amount and will not exceed, in aggregate, $30.0 million, which maximum amount would be owed only if the Company reaches a market capitalization threshold of $9.0 billion and has at least one product candidate covered by a claim of a patent right licensed to the Company under either the Cas9 II License Agreement or the Cas9-I License Agreement that is or was the subject of a clinical trial pursuant to development efforts by the Company or any Company affiliate or sublicensee. The Company triggered the first success payment during the fourth quarter of 2017 when the Company’s market capitalization reached $1.0 billion, which the Company settled in January 2018 (see Note 8).

Common Stock

12 Months Ended

Dec. 31, 2017

Common Stock

Common Stock

11. Common Stock

The voting, dividend, and liquidation rights of the holders of the common stock are subject to and qualified by the rights, powers, and preferences of holders of the preferred stock that may be issued from time to time. The common stock had the following characteristics as of December 31, 2017:

Voting

The holders of shares of common stock are entitled to one vote for each share of common stock held at any meeting of stockholders and at the time of any written action in lieu of a meeting.

Dividends

The holders of shares of common stock are entitled to receive dividends, if and when declared by the Company’s board of directors. Cash dividends may not be declared or paid to holders of shares of common stock until all unpaid dividends on the redeemable convertible preferred stock have been paid in accordance with their terms. No dividends have been declared or paid by the Company since its inception.

Shares Reserved for Future Issuance


	As of December 31,
	2017	2016
Shares reserved for outstanding stock option awards under the 2013 Stock Incentive Plan, as amended	1,220,567	1,595,082
Shares reserved for outstanding stock option awards under the 2015 Stock Incentive Plan	2,921,987	1,569,746
Shares reserved for outstanding inducement stock option award	225,000	—
Remaining shares reserved, but unissued, for future awards under the 2015 Stock Incentive Plan	2,502,338	2,760,472
Remaining shares reserved, but unissued, for future awards under the 2015 Employee Stock Purchase Plan	751,242	384,615
	7,621,134	6,309,915

March 2017 Common Stock Sales Agreement

In March 2017, the Company entered into a sales agreement with Cowen and Company LLC (“Cowen), under which the Company from time to time can issue and sell shares of its common stock through Cowen in at-the-market offerings (“March 2017 ATM Program”) for aggregate sales proceeds of $50.0 million. The common stock will be distributed at the market prices prevailing at the time of sale. All sales of shares will be made pursuant to an effective shelf registration statement on Form S-3 filed with the SEC. The Company will pay Cowen a commission of 3% of the aggregate gross proceeds the Company receives from all sales of the Company’s common stock under the sales agreement. As of December 31, 2017, the Company had not received any proceeds under the March 2017 ATM Program. Subsequent to December 31, 2017 and during January 2018, the Company sold an aggregate of 1,429,205 common shares under the March 2017 ATM Program at an average price of $34.99 per common share for gross proceeds of $50.0 million.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.E)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(e))
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(31))
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 16: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

+ Details

Stock-based Compensation

12 Months Ended

Dec. 31, 2017

Stock-based Compensation

Stock-based Compensation

12. Stock‑Based Compensation

2013 Stock Incentive Plan

In September 2013, the board of directors adopted the 2013 Stock Incentive Plan, as amended (the “2013 Plan”), which provides for the grant of incentive stock options and nonqualified stock options or other awards including restricted stock awards, unrestricted stock awards, and restricted stock units to the Company’s employees, officers, directors, advisors, and consultants for the purchase of up to 1,057,692 shares of the Company’s common stock. In June 2014, the 2013 Plan was amended to increase the number of shares reserved thereunder by 1,365,384 shares. In April 2015, the 2013 Plan was amended to increase the number of shares reserved thereunder by 153,846 shares. In July 2015, the 2013 Plan was amended to increase the number of shares reserved thereunder by 3,740,847 shares.

The terms of stock awards agreements, including vesting requirements, are determined by the board of directors and are subject to the provisions of the 2013 Plan. The stock options granted to employees generally vest over a four-year period and expire ten years from the date of grant. Certain awards contain performance based vesting criteria. There has only been one such award to date. Certain options provide for accelerated vesting in the event of a change in control, as defined in the applicable options. Awards granted to non‑employee consultants generally vest monthly over a period of one to four years. In connection with the IPO, the Company’s board of directors determined to grant no further awards under the 2013 Plan.

2015 Stock Incentive Plan

The Company’s board of directors adopted and the Company’s stockholders approved the 2015 stock incentive plan (the “2015 Plan”), which became effective immediately prior to the effectiveness of the registration statement related to the IPO. The 2015 Plan provides for the grant of incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock‑based awards. The Company’s employees, officers, directors and consultants and advisors are eligible to receive awards under the 2015 Plan.

The number of shares reserved for issuance under the 2015 Plan is subject to further increases for (a) any additional shares of the Company’s common stock subject to outstanding awards under the 2013 Plan that expire, terminate, or are otherwise surrendered, cancelled, forfeited, or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right and (b) annual increases, to be added as of the first day of each fiscal year, from January 1, 2017 until, and including, January 1, 2026, equal to the lowest of 2,923,076 shares of common stock, 4% of the number of shares of common stock outstanding on such first day of the fiscal year in question and an amount determined by the Company’s board of directors. On January 1, 2018, the Company increased the shares under the 2015 Plan by 1,801,017 shares.

2015 Employee Stock Purchase Plan

The Company’s board of directors adopted and the Company’s stockholders approved the 2015 employee stock purchase plan (the “2015 ESPP”), which became effective upon the closing of the IPO. The number of shares reserved for issuance under the 2015 ESPP is subject to annual increases, to be added as of the first day of each fiscal year, from January 1, 2017 until, and including, January 1, 2026, in an amount equal to the least of (a) 769,230 shares of common stock, (b) 1% of the total number of shares of common stock outstanding on the first day of the applicable year, and (c) an amount determined by the board of directors. The first offering under the 2015 ESPP opened on December 1, 2017. On January 1, 2018, the Company increased the shares under the 2015 ESPP Plan by 450,254 shares.

Founder Awards

In September 2013, the Company issued 2,403,845 shares of restricted stock to its non‑employee founders for services rendered subject to certain repurchase rights. The shares vested 25% upon the first issuance of shares of Series A Preferred Stock and then 1.5625% a month through the fourth anniversary of the vesting commencement date. These shares of restricted stock were subject to repurchase rights. Accordingly, the Company recorded the proceeds from the issuance of restricted stock as a liability in its consolidated balance sheets. The restricted stock liability was reclassified into stockholders’ equity (deficit) as the restricted stock vested. In June 2014, one founder ceased to be in the Company’s service and the Company repurchased 285,457 shares of unvested restricted stock from the founder for $74. The remaining founder awards completed vesting in August 2017.

Stock‑based compensation expense associated with these awards was recognized as the awards vested. Unvested awards were remeasured at each reporting period end to reflect the current fair value of such awards on a straight‑line basis.

Stock‑Based Compensation Expense

Total compensation cost recognized for all stock‑based compensation awards in the consolidated statements of operations was as follows (in thousands):


	Year Ended
	December 31,
	2017		2016		2015
Research and development	$	15,131	$	4,234	$	3,015
General and administrative		8,233		12,647		498
Total stock-compensation expense	$	23,364	$	16,881	$	3,513

Restricted Stock

From time to time, upon approval by the Company’s board of directors, certain employees and advisors have been granted restricted shares of common stock. These shares of restricted stock are subject to repurchase rights. Accordingly, the Company has recorded the proceeds from the issuance of restricted stock as a liability in the consolidated balance sheets. The restricted stock liability is reclassified into stockholders’ equity (deficit) as the restricted stock vests. A summary of the status of and changes in unvested restricted stock as of December 31, 2016 and 2017 is as follows:


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested Restricted Common Stock as of December 31, 2016	822,638	$	0.02
Issued	480,000	$	28.05
Vested	(784,119)	$	4.95
Forfeited	(5,294)	$	0.03
Unvested Restricted Common Stock as of December 31, 2017	513,225	$	18.70

The expense related to restricted stock awards granted to employees and non‑employees was $0.5 million and $4.1 million, respectively, for the year ended December 31, 2017. The expense related to restricted stock awards granted to employees and non‑employees was $0 and $8.3 million, respectively, for the year ended December 31, 2016. The expense related to restricted stock awards granted to employees and non-employees was $0 and $2.3 million, respectively, for the year ended December 31, 2015.

As of December 31, 2017, the Company had no unrecognized stock‑based compensation expense related to its employee unvested restricted stock awards. As of December 31, 2017, the Company had unrecognized stock‑based compensation expense related to its non‑employee unvested restricted stock awards of $10.3 million which is expected to be recognized over a remaining weighted average vesting period of 4.7 years.

Stock Options

Certain of the Company’s stock option agreements allow for the exercise of unvested awards. During 2014, options to purchase 75,304 shares of common stock for $0.03 per share were exercised prior to their vesting. The unvested shares are subject to repurchase by the Company if the employees cease to provide service to the Company, with or without cause. As such, the Company does not treat the exercise of unvested options as a substantive exercise. The Company has recorded the proceeds from the exercise of unvested stock options as a liability in the consolidated balance sheets. The liability for unvested common stock subject to repurchase is reclassified into stockholders’ equity (deficit) as the shares vest.

The following is a summary of stock option activity for the year ended December 31, 2017:


		Weighted Average		Remaining	Aggregate Intrinsic
	Shares	Exercise Price		Contractual Life	Value (in thousands)
Outstanding at December 31, 2016	3,411,783	$	13.71	8.8	$	16,190
Granted	1,392,689		—	—		—
Exercised	(289,583)		—	—		—
Cancelled	(142,753)		—	—		—
Outstanding at December 31, 2017	4,372,136	$	17.28	8.5	$	60,591
Vested and expected to vest at December 31, 2017	4,372,126	$	17.28	8.5	$	60,591
Exercisable at December 31, 2017	1,540,023	$	14.95	8.3	$	25,099

The table above reflects unvested stock options as exercised on the dates that the shares are no longer subject to repurchase. The Company had 4,572 and 21,955 shares of unvested common stock at December 31, 2017 and 2016 related to the exercise of unvested stock options.

The total intrinsic value of options exercised for the years ended December 31, 2017, 2016, and 2015 was $5.0 million, $0.9 million, and $0.1 million, respectively.

Using the Black‑Scholes option pricing model, the weighted average fair value of options granted to employees and directors during the years ended December 31, 2017, 2016, and 2015 was $16.07, $14.10, and $5.91, respectively. The expense related to options granted to employees and directors was $12.3 million, $6.0 million, and $0.7 million for the years ended December 31, 2017, 2016, and 2015, respectively.

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:


	Year Ended
	December 31,
	2017		2016		2015
Expected volatility	77.8	%	78.4	%	78.8	%
Expected term (in years)	6.25		6.25		6.25
Risk free interest rate	2.1	%	1.5	%	1.7	%
Expected dividend yield	—		—		—

There were no options granted to persons other than employees and directors during the year ended December 31, 2017. For the year ended December 31, 2017, 2016 and 2015, the fair value of each option issued to persons other than employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the weighted‑average assumptions set forth in the table below:


	Year Ended
	December 31,
	2017	2016		2015
Expected volatility	—	76.5	%	80.0	%
Expected term (in years)	—	10.0		10.0
Risk free interest rate	—	1.6	%	2.2	%
Expected dividend yield	—	—		—

As of December 31, 2017, the Company had unrecognized stock‑based compensation expense related to its employee stock options of $33.4 million which the Company expects to recognize over a remaining weighted average vesting period of 2.5 years.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

401(k) Savings Plan	12 Months Ended
401(k) Savings Plan	Dec. 31, 2017
401(k) Savings Plan
401(k) Savings Plan	13. 401(k) Savings Plan The Company has a defined‑contribution savings plan under Section 401(k) of the Internal Revenue Code of 1986, as amended (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. Effective in 2017, the Company will provide a 200% match of employee contributions up to a limit on the Company’s contributions of the lesser of $6,000 and 3% of the employee’s salary. The Company made $0.5 million in contributions to the 401(k) Plan for the year ended December 31, 2017 and did not make any contributions for the years ended December 31, 2016 and 2015, respectively.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Taxes

Income Taxes

14. Income Taxes

A reconciliation of the income tax expense computed using the federal statutory income tax rate to the Company’s effective income tax rate is as follows:


	Year Ended
	December 31,
	2017		2016		2015
Income tax computed at federal statutory tax rate	34.0	%	34.0	%	34.0	%
State taxes, net of federal benefit	5.9	%	3.5	%	2.5	%
General business credit carryovers	2.5	%	1.5	%	0.8	%
Non-deductible expenses	(2.1)	%	(3.6)	%	(17.9)	%
Federal tax rate reduction	(24.7)	%	—	%	—	%
Change in valuation allowance	(15.6)	%	(35.4)	%	(19.4)	%
	—	%	—	%	—	%

On December 22, 2017, legislation commonly known as the Tax Cuts and Jobs Act (the “Tax Act”) was signed into law. The Tax Act, among other changes, reduces the U.S. federal corporate tax rate from 34% to 21%, requires taxpayers to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. As of December 31, 2017, the Company did not have any foreign subsidiaries and the international aspects of the Tax Act are not applicable.

In connection with the initial analysis on the impact of the Tax Act, the Company remeasured certain deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21%. The remeasurement of the Company's deferred tax balance was primarily offset by application of its valuation allowance. However, the reduction of the U.S. federal corporate tax rate resulted in increases to the amounts reflected in “Federal tax rate reduction” and “Change in valuation allowance” captions for the year ended December 31, 2017 in the Company’s tax reconciliation table compared to those amounts disclosed for the years ended December 31, 2016 and 2015. The change in the U.S. federal corporate tax rate, which is effective January 1, 2018, is also reflected in the Company’s deferred tax table.

The Company is still in the process of analyzing the impact to the Company of the Tax Act. On December 22, 2017, the SEC staff issued SAB 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Act. The Company has recognized the provisional tax impacts related to the revaluation of the deferred tax assets and liabilities and included these amounts in its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of the Tax Act, which could result in changes to the provisional tax impacts during 2018.

The principal components of the Company’s deferred tax assets and liabilities consist of the following at December 31, 2017 and 2016 (in thousands):


	Year Ended
	December 31,
	2017		2016
Deferred tax assets:
Net operating loss carryforwards	$	27,726	$	16,490
Tax credit carryforwards		5,259		2,014
Accrued expenses		2,079		7,353
Capitalized patent costs		26,307		16,025
Deferred revenue		7,151		9,672
Construction financing lease obligation		9,352		13,685
Other		4,978		2,979
Total deferred tax assets		82,852		68,218
Less valuation allowance		(73,301)		(54,300)
Net deferred tax assets		9,551		13,918
Deferred tax liabilities—depreciation and amortization		(9,551)		(13,918)
Net deferred taxes	$	—	$	—

The Company has incurred net operating losses (“NOL”) since inception. At December 31, 2017 and 2016, the Company had federal and state net operating loss carryforwards of $202.7 million and $82.4 million respectively, which expire beginning in 2033 and will continue to expire through 2037. As of December 31, 2017 and 2016, the Company had federal and state research and development tax credits carryforwards of $5.6 million and $2.3 million, respectively, which expire beginning in 2028 and will continue to expire through 2037.

Under the provisions of the Internal Revenue Code of 1986, as amended (the “Code”), the NOL and tax credit carryforward are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three year period in excess of 50%, as defined under Sections 382 and 383 of the Code, respectively, as well as other similar state provisions. The Company has not performed a full comprehensive Section 382 study to determine any potential loss limitation in the United States or a Section 383 study to determine the appropriate amount of NOL and tax credit carryforwards.

Management has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which principally comprise of NOL carryforwards, research and development credit carryforwards and capitalized license and patent costs. Management has determined that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets, and as a result, a valuation allowance of $73.3 million and $54.3 million has been established at December 31, 2017 and 2016, respectively. The increase in the valuation allowance of $19.0 million for the year ended December 31, 2017 was primarily due to current year operating losses offset by the federal rate reduction from 34% to 21% as a result of the Tax Act.

The Company applies ASC 740 related to accounting for uncertainty in income taxes. The Company’s reserves related to income taxes are based on a determination of whether, and how much of, a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. At December 31, 2017 and 2016, the Company had no unrecognized tax benefits. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statements of operations.

The Company has not as yet conducted a study of its research and development credit carry forwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the consolidated balance sheets or statements of operations if an adjustment were required.

The Company files income tax returns in the U.S. federal tax jurisdiction, the Massachusetts state jurisdiction and the California state jurisdiction. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available. The Company did not have any international operations as of December 31, 2017. There are no federal or state audits in process.

Net Loss per Share

12 Months Ended

Dec. 31, 2017

Net Loss per Share

Net Loss per Share

15. Net Loss per Share

Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted average number of common shares and potentially dilutive securities outstanding for the period determined using the treasury stock and if converted methods. Contingently issuable shares are included in the calculation of basic loss per share as of the beginning of the period in which all the necessary conditions have been satisfied. Contingently issuable shares are included in diluted loss per share based on the number of shares, if any, that would be issuable under the terms of the arrangement if the end of the reporting period was the end of the contingency period, if the results are dilutive.

For purposes of the diluted net loss per share calculation, stock options are considered to be common stock equivalents, but they were excluded from the Company’s calculation of diluted net loss per share allocable to common stockholders because their inclusion would have been anti-dilutive. Therefore, basic and diluted net loss per share applicable to common stockholders was the same for all periods presented.

Upon the closings of the March Offering and the December Offering, the Company sold 4,600,000 and 2,265,500 shares of common stock, respectively. The issuance of these shares resulted in a significant increase in the Company’s weighted-average shares outstanding for the year ended December 31, 2017 when compared to the comparable prior year period and is expected to continue to impact the year-over-year comparability of the Company’s net loss per share calculations for the next three and twelve months.

The following common stock equivalents were excluded from the calculation of diluted net loss per share allocable to common stockholders because their inclusion would have been anti-dilutive:


	As of December 31,
	2017	2016
Unvested restricted common stock	513,225	822,638
Outstanding stock options	4,372,126	3,411,783
Estimated number of shares issuable for convertible notes (1)	244,896	—
Total	5,130,247	4,234,421

(1)

Represents the number of shares that would have been issued if the Company had elected to pay the December Success Payment Notes, as discussed in Note 8, in shares of the Company’s common stock, based on the closing price of the common stock on December 31, 2017. The number of shares issued, for purposes of this presentation, is calculated by dividing the principal of the notes payable, including accrued interest, by the stock price per share.

The table above reflects restricted stock issued upon exercise of unvested stock options as exercised on the dates that the shares are no longer subject to repurchase.

Related-party Transactions	12 Months Ended
Related-party Transactions	Dec. 31, 2017
Related-party Transactions
Related-party Transactions	16. Related‑Party Transactions During the years ended December 31, 2016 and December 31, 2015, the Company paid a related party $1.4 million and $1.2 million in rent and facility-related fees, respectively. The Company did not make any payments to this related party during the year ended December 31, 2017. The Company received $0.8 million in rent and facility-related fees from a related party during the year ended December 31, 2017 in connection with the Sublease; no rent or facility-related payments were received from this related party during the year ended December 31, 2016 or December 31, 2015. In addition, during the year ended December 31, 2015, the Company paid one of its investors $0.1 million in professional fees.

Selected Quarterly Financial Data

12 Months Ended

Dec. 31, 2017

Selected Quarterly Financial Data

Selected Quarterly Financial Data

17. Selected Quarterly Financial Data (unaudited) –

The following table contains selected quarterly financial information from 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.


	Three months ended
	March 31, 2017		June 30, 2017		September 30, 2017		December 31, 2017
		(in thousands, except per share data)
Total revenue	$	682	$	3,097	$	6,282	$	3,667
Total operating expenses		31,309		29,212		33,031		40,109
Total other income (expense), net		(470)		(324)		150		253
Net loss	$	(31,097)	$	(26,439)	$	(26,599)	$	(36,189)
Net loss applicable to common stockholders	$	(31,097)	$	(26,439)	$	(26,599)	$	(36,189)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.85)	$	(0.65)	$	(0.64)	$	(0.84)


	Three months ended
	March 31, 2016		June 30, 2016		September 30, 2016		December 31, 2016
		(in thousands, except per share data)
Total revenue	$	805	$	3,388	$	962	$	898
Total operating expenses		18,644		22,588		22,127		39,882
Total other income (expense), net		94		158		145		(392)
Net loss	$	(17,745)	$	(19,042)	$	(21,020)	$	(39,376)
Net loss applicable to common stockholders	$	(17,792)	$	(19,042)	$	(21,020)	$	(39,376)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.80)	$	(0.54)	$	(0.59)	$	(1.10)

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2018
Subsequent Events
Subsequent Events	18. Subsequent Events In January 2018, the Company issued an aggregate of 225,909 shares of its common stock to Broad as payment of all outstanding principal and interest under the December Success Payment Notes (see Note 8). Upon such issuance and payment, the December Success Payment Notes were cancelled. In January 2018, the Company issued an aggregate of 80,000 shares of its common stock to MGH in connection with settling the First MGH Success Payment (see Note 8). In January 2018, the Company paid $1.6 million in cash and issued an aggregate of 56,099 shares of its common stock, valued at $29.76 per share, to i2 Pharmaceuticals, Inc. in connection with the purchase of certain assets pursuant to an asset purchase agreement. In January 2018, the Company sold an aggregate of 1,429,205 common shares under the March 2017 ATM Program at an average price of $34.99 per common share for gross proceeds of $50.0 million. The March 2017 ATM Program was fully utilized in January 2018.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Principles of Consolidation

Basis of Presentation

Reclassification

Certain prior period amounts have been reclassified for consistency with the current period presentation. These reclassifications had no effect on previously reported results of operations.

Use of Estimates

Fair Value of Financial Instruments

Level 1 – Quoted market prices in active markets for identical assets or liabilities.

Level 2 – Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves.

Level 3 – Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

Cash, Cash Equivalents, and Restricted Cash


	Year Ended
	As of December 31,
	2017		2016		2015
Cash and cash equivalents	$	146,630	$	185,323	$	143,180
Restricted cash included in "Prepaid expenses and other current assets"		—		—		320
Restricted cash included in "Restricted cash and other non-current assets"		1,619		1,619		—
Total	$	148,249	$	186,942	$	143,500

Marketable Securities

Accounts Receivable

Property and Equipment


Asset:		Estimated Useful life
Lab equipment		5 years
Computer equipment and software		3 years
Furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or remaining lease term
Building		30 years

Impairment of Long-lived Assets

Impairment of Long‑lived Assets

Revenue Recognition

To date, the Company has primarily earned revenue under the collaboration and license agreement with Juno Therapeutics and the strategic research alliance with Allergan (see Note 9).

Persuasive evidence of an arrangement exists;

Delivery has occurred or services have been rendered;

The seller’s price to the buyer is fixed or determinable; and

Collectability is reasonably assured.

Research and Development Costs

Patent Costs

Construction Financing Lease Obligation

Stock-based Compensation Expense

Stock‑based Compensation Expense

Determination of Fair Value of Common Stock on Grant Dates prior to our Initial Public Offering

Income taxes

Other Income (Expense), Net

Comprehensive Loss

Concentrations of Credit Risk and Off-Balance Sheet Risk

Concentrations of Credit Risk and Off‑Balance Sheet Risk

Segment Information

Recent Accounting Pronouncements

Net loss per share

Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Schedule of cash, cash equivalents, and restricted cash


	Year Ended
	As of December 31,
	2017		2016		2015
Cash and cash equivalents	$	146,630	$	185,323	$	143,180
Restricted cash included in "Prepaid expenses and other current assets"		—		—		320
Restricted cash included in "Restricted cash and other non-current assets"		1,619		1,619		—
Total	$	148,249	$	186,942	$	143,500

Schedule of estimated useful lives of property, plant and equipment


Asset:		Estimated Useful life
Lab equipment		5 years
Computer equipment and software		3 years
Furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or remaining lease term
Building		30 years

Cash Equivalents & Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2017

Cash Equivalents & Marketable Securities

Schedule of cash equivalents and marketable securities

Cash equivalents and marketable securities consisted of the following at December 31, 2017 (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Cost		Gains		Losses		Value
Cash equivalents:
Money market funds	$	134,635	$	—	$	—	$	134,635
U.S. Treasuries		11,995		—		—		11,995
Marketable securities:
U.S. Treasuries		123,606		—		(47)		123,559
Government agency securities		58,979		—		(29)		58,950
Total cash equivalents and marketable securities	$	329,215	$	—	$	(76)	$	329,139

Cash equivalents and marketable securities consisted of the following at December 31, 2016 (in thousands):


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
December 31, 2016	Cost		Gains		Losses		Value
Cash equivalents:
Money market funds	$	185,323	$	—	$	—	$	185,323
Total cash equivalents and marketable securities	$	185,323	$	—	$	—	$	185,323

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Measurements

Schedule of assets measured at fair value on a recurring basis

Assets measured at fair value on a recurring basis as of December 31, 2017 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	December 31,		Identical Assets		Inputs		Inputs
Financial Assets	2017		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents
Money market funds	$	134,635	$	134,635	$	—	$	—
U.S. Treasuries		11,995		11,995		—		—
Marketable securities:
U.S. Treasuries		123,559		123,559		—		—
Government agency securities		58,950		58,950		—		—
Money market funds, included in restricted cash		1,619		1,619		—		—
Total financial assets	$	330,758	$	330,758	$	—	$	—

Assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	December 31,		Identical Assets		Inputs		Inputs
Financial Assets	2016		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents	$	185,323	$	185,323	$	—	$	—
Money market funds, included in other current assets		1,619		1,619		—		—
Total financial assets	$	186,942	$	186,942	$	—	$	—

Prepaid Expenses and Other Current Assets (Tables)

12 Months Ended

Dec. 31, 2017

Prepaid Expenses and Other Current Assets

Schedule of prepaid expenses and other current assets

Prepaid expense and other current assets consisted of the following (in thousands):


	As of
	December 31,
	2017		2016
Prepaid expenses	$	1,864	$	1,662
Other		517		110
Total	$	2,381	$	1,772

Property and Equipment, net (Tables)

12 Months Ended

Dec. 31, 2017

Property and Equipment, net

Schedule of property and equipment, net

Property and equipment, net consisted of the following (in thousands):


	As of
	December 31,
	2017		2016
Building	$	35,167	$	35,941
Laboratory equipment		7,415		5,130
Computer equipment		550		392
Leasehold improvements		177		200
Furniture and office equipment		96		170
Software		95		101
Total property and equipment		43,500		41,934
Less: accumulated depreciation		(4,058)		(1,556)
Property and equipment, net	$	39,442	$	40,378

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Accrued Expenses

Schedule of accrued expenses

Accrued expenses consisted of the following (in thousands):


	As of
	December 31,
	2017		2016
Employee related expenses	$	3,708	$	2,480
Intellectual property and patent related fees		2,370		13,251
Process and platform development expenses		2,301		443
Success payment expenses		2,000		—
Professional service expenses		487		729
Other expenses		183		536
Total	$	11,049	$	17,439

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Schedule of future annual minimum lease payments

The non-cancelable minimum annual lease payments, excluding the Company’s share of the facility operating expenses and other costs that are reimbursable to the landlord under the lease, consist of the following (in thousands):


Year ended December 31,	11 Hurley Street Lease
2018		4,055
2019		4,155
2020		4,257
2021		4,362
2022		4,470
2023 and thereafter		3,801
Total minimum lease payments	$	25,100

Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Common Stock

Schedule of shares reserved for future issuance


	As of December 31,
	2017	2016
Shares reserved for outstanding stock option awards under the 2013 Stock Incentive Plan, as amended	1,220,567	1,595,082
Shares reserved for outstanding stock option awards under the 2015 Stock Incentive Plan	2,921,987	1,569,746
Shares reserved for outstanding inducement stock option award	225,000	—
Remaining shares reserved, but unissued, for future awards under the 2015 Stock Incentive Plan	2,502,338	2,760,472
Remaining shares reserved, but unissued, for future awards under the 2015 Employee Stock Purchase Plan	751,242	384,615
	7,621,134	6,309,915

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Stock-based Compensation

Schedule of stock-based compensation expense

Total compensation cost recognized for all stock‑based compensation awards in the consolidated statements of operations was as follows (in thousands):


	Year Ended
	December 31,
	2017		2016		2015
Research and development	$	15,131	$	4,234	$	3,015
General and administrative		8,233		12,647		498
Total stock-compensation expense	$	23,364	$	16,881	$	3,513

Schedule of changes in unvested restricted stock


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested Restricted Common Stock as of December 31, 2016	822,638	$	0.02
Issued	480,000	$	28.05
Vested	(784,119)	$	4.95
Forfeited	(5,294)	$	0.03
Unvested Restricted Common Stock as of December 31, 2017	513,225	$	18.70

Schedule of stock option activity


		Weighted Average		Remaining	Aggregate Intrinsic
	Shares	Exercise Price		Contractual Life	Value (in thousands)
Outstanding at December 31, 2016	3,411,783	$	13.71	8.8	$	16,190
Granted	1,392,689		—	—		—
Exercised	(289,583)		—	—		—
Cancelled	(142,753)		—	—		—
Outstanding at December 31, 2017	4,372,136	$	17.28	8.5	$	60,591
Vested and expected to vest at December 31, 2017	4,372,126	$	17.28	8.5	$	60,591
Exercisable at December 31, 2017	1,540,023	$	14.95	8.3	$	25,099

Schedule of assumptions used to value stock options

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:


	Year Ended
	December 31,
	2017		2016		2015
Expected volatility	77.8	%	78.4	%	78.8	%
Expected term (in years)	6.25		6.25		6.25
Risk free interest rate	2.1	%	1.5	%	1.7	%
Expected dividend yield	—		—		—


	Year Ended
	December 31,
	2017	2016		2015
Expected volatility	—	76.5	%	80.0	%
Expected term (in years)	—	10.0		10.0
Risk free interest rate	—	1.6	%	2.2	%
Expected dividend yield	—	—		—

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Taxes

Schedule of reconciliation of effective income tax rate

A reconciliation of the income tax expense computed using the federal statutory income tax rate to the Company’s effective income tax rate is as follows:


	Year Ended
	December 31,
	2017		2016		2015
Income tax computed at federal statutory tax rate	34.0	%	34.0	%	34.0	%
State taxes, net of federal benefit	5.9	%	3.5	%	2.5	%
General business credit carryovers	2.5	%	1.5	%	0.8	%
Non-deductible expenses	(2.1)	%	(3.6)	%	(17.9)	%
Federal tax rate reduction	(24.7)	%	—	%	—	%
Change in valuation allowance	(15.6)	%	(35.4)	%	(19.4)	%
	—	%	—	%	—	%

Schedule of components of deferred tax assets and liabilities

The principal components of the Company’s deferred tax assets and liabilities consist of the following at December 31, 2017 and 2016 (in thousands):


	Year Ended
	December 31,
	2017		2016
Deferred tax assets:
Net operating loss carryforwards	$	27,726	$	16,490
Tax credit carryforwards		5,259		2,014
Accrued expenses		2,079		7,353
Capitalized patent costs		26,307		16,025
Deferred revenue		7,151		9,672
Construction financing lease obligation		9,352		13,685
Other		4,978		2,979
Total deferred tax assets		82,852		68,218
Less valuation allowance		(73,301)		(54,300)
Net deferred tax assets		9,551		13,918
Deferred tax liabilities—depreciation and amortization		(9,551)		(13,918)
Net deferred taxes	$	—	$	—

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2017

Net Loss per Share

Schedule of anti-dilutive common stock equivalents

The following common stock equivalents were excluded from the calculation of diluted net loss per share allocable to common stockholders because their inclusion would have been anti-dilutive:


	As of December 31,
	2017	2016
Unvested restricted common stock	513,225	822,638
Outstanding stock options	4,372,126	3,411,783
Estimated number of shares issuable for convertible notes (1)	244,896	—
Total	5,130,247	4,234,421

(1)

Selected Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2017

Selected Quarterly Financial Data

Schedule of selected quarterly financial information


	Three months ended
	March 31, 2017		June 30, 2017		September 30, 2017		December 31, 2017
		(in thousands, except per share data)
Total revenue	$	682	$	3,097	$	6,282	$	3,667
Total operating expenses		31,309		29,212		33,031		40,109
Total other income (expense), net		(470)		(324)		150		253
Net loss	$	(31,097)	$	(26,439)	$	(26,599)	$	(36,189)
Net loss applicable to common stockholders	$	(31,097)	$	(26,439)	$	(26,599)	$	(36,189)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.85)	$	(0.65)	$	(0.64)	$	(0.84)


	Three months ended
	March 31, 2016		June 30, 2016		September 30, 2016		December 31, 2016
		(in thousands, except per share data)
Total revenue	$	805	$	3,388	$	962	$	898
Total operating expenses		18,644		22,588		22,127		39,882
Total other income (expense), net		94		158		145		(392)
Net loss	$	(17,745)	$	(19,042)	$	(21,020)	$	(39,376)
Net loss applicable to common stockholders	$	(17,792)	$	(19,042)	$	(21,020)	$	(39,376)
Net loss per share applicable to common stockholders — basic and diluted	$	(0.80)	$	(0.54)	$	(0.59)	$	(1.10)

Nature of Business (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Feb. 29, 2016	Dec. 31, 2016
Liquidity
Accumulated deficit	$ (305,850)			$ (185,526)
Initial Public Offering
Liquidity
Number of common stock issued	2,265,500	4,600,000	6,785,000
Price to the public			$ 16.00
Aggregate net proceeds			$ 97,500
Follow-on Offering
Liquidity
Number of common stock issued	295,500	4,600,000
Price to the public		$ 22.50
Aggregate net proceeds		$ 96,700
Public Offering
Liquidity
Number of common stock issued	2,265,500
Price to the public	$ 26.00
Aggregate net proceeds	$ 57,200
Overallotment Option
Liquidity
Number of common stock issued		600,000	885,000

Summary Of Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash, Cash Equivalents, and Restricted Cash
Restricted cash	$ 1,600	$ 1,600	$ 300
Cash and cash equivalents	146,630	185,323	143,180
Restricted cash included in "Prepaid expenses and other current assets"			320
Restricted cash included in "Restricted cash and other non-current assets"	1,619	1,619
Total	$ 148,249	$ 186,942	$ 143,500	$ 10,983

Summary Of Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable
Bad debt write-offs	$ 0	$ 0
Allowance for doubtful accounts	$ 0	$ 0

Summary Of Significant Accounting Policies - Property And Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and equipment policy
Impairment losses	$ 0	$ 0	$ 0
Laboratory equipment
Property and equipment policy
Estimated useful life	5 years
Computer equipment and software
Property and equipment policy
Estimated useful life	3 years
Furniture and office equipment
Property and equipment policy
Estimated useful life	5 years
Building
Property and equipment policy
Estimated useful life	30 years

Summary Of Significant Accounting Policies - Warrants to Purchase Convertible Preferred Stock (Details)	1 Months Ended		3 Months Ended								10 Months Ended	12 Months Ended			20 Months Ended	32 Months Ended
	Jul. 31, 2017 USD ($)	May 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Recent Accounting Pronouncements
Number of Revenue Agreements \| item												3
Collaboration and other research and development revenues			$ 3,667,000	$ 6,282,000	$ 3,097,000	$ 682,000	$ 898,000	$ 962,000	$ 3,388,000	$ 805,000		$ 13,728,000	$ 6,053,000	$ 1,629,000
Adjustment to valuation allowance												19,000,000
ASU 2016-09
Recent Accounting Pronouncements
Adjustment to valuation allowance												179,000
Juno Therapeutics \| Collaboration and License Agreement
Recent Accounting Pronouncements
Collaboration and other research and development revenues												4,900,000	$ 5,700,000	$ 1,600,000		$ 12,200,000
Milestone payment received under license agreement	$ 2,500,000	$ 2,500,000
Cystic Fibrosis Foundation Therapeutics, Inc. \| Development award agreement
Recent Accounting Pronouncements
Collaboration and other research and development revenues															$ 300,000
Allergan \| Strategic Alliance
Recent Accounting Pronouncements
Collaboration and other research and development revenues											$ 8,800,000	$ 8,800,000

Cash Equivalents & Marketable Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Cash equivalents and marketable securities
Amortized Cost	$ 329,215	$ 185,323
Gross Unrealized Losses	(76)
Fair Value	$ 329,139	185,323
Number of securities in an unrealized loss position \| item	25
Securities held by the Company in an unrealized loss position for less than 12 months	$ 174,000
Number of securities in an unrealized loss position for more than 12 months \| item	0
Realized gains (losses) on available-for-sale securities	$ 0	0
U.S. Treasuries
Cash equivalents and marketable securities
Amortized Cost	123,606
Gross Unrealized Losses	(47)
Fair Value	123,559
Government agency securities
Cash equivalents and marketable securities
Amortized Cost	58,979
Gross Unrealized Losses	(29)
Fair Value	58,950
Money market funds
Cash equivalents and marketable securities
Amortized Cost	134,635	185,323
Fair Value	134,635	$ 185,323
U.S. Treasuries
Cash equivalents and marketable securities
Amortized Cost	11,995
Fair Value	$ 11,995

Fair Value Measurements - Recurring Basis (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Financial Assets
Marketable securities	$ 329,139	$ 185,323
Transfers between levels \| item	0	0
U.S. Treasuries
Financial Assets
Marketable securities	$ 123,559
Government agency securities
Financial Assets
Marketable securities	58,950
Money market funds
Financial Assets
Marketable securities	134,635	$ 185,323
U.S. Treasuries
Financial Assets
Marketable securities	11,995
Recurring
Financial Assets
Cash and cash equivalents		185,323
Total financial assets	330,758	186,942
Recurring \| U.S. Treasuries
Financial Assets
Marketable securities	123,559
Recurring \| Government agency securities
Financial Assets
Marketable securities	58,950
Recurring \| Money market funds
Financial Assets
Cash and cash equivalents	134,635
Restricted cash	1,619	1,619
Recurring \| U.S. Treasuries
Financial Assets
Cash and cash equivalents	11,995
Recurring \| Level 1
Financial Assets
Cash and cash equivalents		185,323
Total financial assets	330,758	186,942
Recurring \| Level 1 \| U.S. Treasuries
Financial Assets
Marketable securities	123,559
Recurring \| Level 1 \| Government agency securities
Financial Assets
Marketable securities	58,950
Recurring \| Level 1 \| Money market funds
Financial Assets
Cash and cash equivalents	134,635
Restricted cash	1,619	$ 1,619
Recurring \| Level 1 \| U.S. Treasuries
Financial Assets
Cash and cash equivalents	$ 11,995

Prepaid Expenses And Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid Expenses and Other Current Assets
Prepaid expenses	$ 1,864	$ 1,662
Other	517	110
Total	$ 2,381	$ 1,772

Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and equipment disclosures
Total property and equipment	$ 43,500	$ 41,934
Less: accumulated depreciation	(4,058)	(1,556)
Property and equipment, net	39,442	40,378
Depreciation expense	2,683	1,202	$ 471
Building
Property and equipment disclosures
Total property and equipment	35,167	35,941
Laboratory equipment
Property and equipment disclosures
Total property and equipment	7,415	5,130
Computer equipment
Property and equipment disclosures
Total property and equipment	550	392
Leasehold improvements
Property and equipment disclosures
Total property and equipment	177	200
Furniture and office equipment
Property and equipment disclosures
Total property and equipment	96	170
Software
Property and equipment disclosures
Total property and equipment	$ 95	$ 101

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses
Employee related expenses	$ 3,708	$ 2,480
Intellectual property and patent related fees	2,370	13,251
Process and platform development expenses	2,301	443
Success payment expenses	2,000
Professional service expenses	487	729
Other expenses	183	536
Total	$ 11,049	$ 17,439

Commitments and Contingencies - Operating Leases (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2017 ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 29, 2016 USD ($) ft²
Future annual minimum lease payments
Rent expense		$ 1,200	$ 2,500	$ 1,000
Hurley Street Lease
Commitments and contingencies
Leased space ( in square feet) \| ft²					59,783
Security deposit					$ 1,600
Estimated useful life			30 years
Extended lease option (in years)		5 years
Future annual minimum lease payments
2018		$ 4,055
2019		4,155
2020		4,257
2021		4,362
2022		4,470
2023 and after		3,801
Total minimum lease payments		25,100
Facility Sublease Arrangement
Commitments and contingencies
Leased space ( in square feet) \| ft²	10,000
Future minimum rental revenue to be received		$ 400
Sublease term	18 months
Lease extension term	18 months

Commitments and Contingencies - Notes Payable (Details) - USD ($) $ in Thousands			1 Months Ended						12 Months Ended
	Dec. 06, 2017	Mar. 28, 2017	Jan. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Aug. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017
Commitments and contingencies
Accrued expenses				$ 11,049				$ 17,439	$ 11,049
Notes Payable
Notes paid in cash									600
2016 License Agreements
Commitments and contingencies
Market Capitalization				1,000,000			$ 750,000		1,000,000
Broad and Wageningen University \| Cpf1 License Agreement
Notes Payable
Promissory notes issued								$ 10,000
Interest rate (as a percentage)								4.80%
Success Payments							5,000
Broad and Wageningen University \| Cpf1 License Agreement \| Success Payment Notes
Notes Payable
Promissory notes issued							$ 5,000
The Broad Institute Inc \| 2016 License Agreements
Notes Payable
Promissory notes issued	$ 7,500	$ 5,000
The Broad Institute Inc \| Cpf1 License Agreement
Notes Payable
Common stock issued					371,166	108,104
Success Payments				$ 7,500
The Broad Institute Inc \| Cpf1 License Agreement \| Success Payment Notes
Notes Payable
Promissory notes issued									7,500
Common stock issued						271,347
The Broad Institute Inc \| Cpf1 License Agreement \| Subsequent Event \| Success Payment Notes
Notes Payable
Common stock issued			225,909
Wageningen University \| 2016 License Agreements
Notes Payable
Promissory notes issued		$ 5,000
Wageningen University \| Cpf1 License Agreement
Notes Payable
Notes paid in cash						$ 200
Wageningen University \| Cpf1 License Agreement \| Success Payment Notes
Notes Payable
Notes paid in cash						$ 400
The General Hospital (MGH) \| 2016 License Agreements
Notes Payable
Success Payments									$ 2,000
The General Hospital (MGH) \| 2016 License Agreements \| Subsequent Event
Notes Payable
Common stock issued			80,000

Significant Agreements - Juno Therapeutics Collaboration Agreement (Details) $ in Thousands	1 Months Ended				3 Months Ended								10 Months Ended	12 Months Ended			32 Months Ended
	Jul. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) item	May 31, 2016 USD ($)	May 31, 2015 USD ($) item	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)
Collaboration and other research and development revenues					$ 3,667	$ 6,282	$ 3,097	$ 682	$ 898	$ 962	$ 3,388	$ 805		$ 13,728	$ 6,053	$ 1,629
Advance payment on future research and development activities					1,864				1,662				$ 1,864	1,864	1,662		$ 1,864
Research and development														83,159	56,979	18,846
Deferred revenue, long-term					94,725				26,000				94,725	94,725	26,000		94,725
Accounts receivable					679				88				679	$ 679	88		679
Allergan
Number of additional licenses \| item														2
One time payment to acquire the license					25,000								25,000	$ 25,000			25,000
Potential development milestone payments					8,000								8,000	$ 8,000			8,000
Number of days in which payment to be made														45 days
Allergan \| Maximum
Potential development milestone payments					42,000								42,000	$ 42,000			42,000
Potential regulatory and commercial milestone payments					75,000								75,000	75,000			75,000
Potential commercial milestone payments					90,000								90,000	90,000			90,000
Collaboration and License Agreement \| Juno Therapeutics
Number of research areas \| item				3
Agreement term				5 years
Extensions \| item				2
Extension period				1 year
Upfront fee received				$ 25,000
Potential research and development funding				22,000
Potential fee receivable for each gene target licensed				$ 2,500
Number of additional licenses \| item				3
Potential development milestone payments				$ 77,500
Potential regulatory milestone payments				80,000
Potential commercial milestone payments				$ 75,000
Milestone payment received under license agreement	$ 2,500		$ 2,500
Next possible milestone payment					2,500								2,500	2,500			2,500
Number of options to purchase Development and Commercialization License \| item				3
Number of deliverables \| item				6
Number of accounting units \| item				11
Number of accounting units for Development and Commercialization License \| item				3
Number of accounting units for discount deliverables \| item				6
Allocable arrangement consideration - upfront payment				$ 25,000
Allocable arrangement consideration - research support				20,000
Allocable arrangement consideration - specialized material costs				2,000
Allocable arrangement consideration - aggregate arrangement consideration				47,000
Allocable arrangement consideration - R&D Services Unit of Accounting				16,700
Allocable arrangement consideration - Development and Commercialization License for the first research area				9,300
Allocable arrangement consideration - Development and Commercialization License for the second research area				15,400
Allocable arrangement consideration - Development and Commercialization License for the third research area				200
Allocable arrangement consideration - first Discount Deliverable for the first research area				700
Allocable arrangement consideration - second Discount Deliverable for the first research area				400
Allocable arrangement consideration - third Discount Deliverable for the first research area				200
Allocable arrangement consideration - first Discount Deliverable for the second research area				2,000
Allocable arrangement consideration - second Discount Deliverable for the second research area				1,300
Allocable arrangement consideration - third Discount Deliverable for the second research area				800
Allocable arrangement consideration -JRC Deliverable				$ 0
Collaboration and other research and development revenues														4,900	5,700	$ 1,600	12,200
Research and development														500	500
Deferred revenue					26,400				26,000				26,400	26,400	26,000		26,400
Accounts receivable					500				$ 0				500	500	$ 0		500
Strategic Alliance \| Allergan
Agreement term		7 years
Number of Collaboration Development Programs \| item		5
Number of Committee Members \| item		3
Extension period		1 year
Upfront fee received		$ 90,000
Option exercise price per CDP		15,000						15,000
Extension fee for Initial Option Period		5,000						5,000
Option exercise price for development and commercialization license		22,500						22,500
Allocable arrangement consideration - aggregate arrangement consideration		$ 90,000						$ 90,000
Collaboration and other research and development revenues													8,800	8,800
Research and development														14,100
Deferred revenue					81,200								81,200	81,200			81,200
Deferred revenue, long-term					$ 68,300								$ 68,300	$ 68,300			$ 68,300
Contract termination notice														90 days
Strategic Alliance \| Allergan \| Maximum
Extension period		10 years

Significant Agreements - Other Agreements (Details) $ in Thousands	1 Months Ended							12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 shares	Aug. 31, 2017 shares	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($) item	Oct. 31, 2014 USD ($) shares	Aug. 31, 2014 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2017 USD ($)	Dec. 16, 2016 USD ($)
Notes payable	$ 7,500			$ 10,000				$ 7,500	$ 10,000
Research and development								83,159	$ 56,979	$ 18,846
The General Hospital (MGH) \| License Patent Rights
Upfront fee							$ 100
Stock issued to licensors \| shares							66,848
Percentage of outstanding shares issued							0.50%
Research and development					$ 800
Market capitalization threshold					1,000,000
Market capitalization	1,000,000							$ 1,000,000
The General Hospital (MGH) \| Maximum \| License Patent Rights
Potential liability for future clinical, regulatory and commercial milestone payments related to product approval					1,000		$ 3,700
Potential liability for future sales milestone payments							$ 1,800
Potential liability for future clinical and regulatory milestone payments related to product approval					$ 4,900
Percentage of net sales threshold for potential milestone payments to be made					1.00%
Number of licensed products \| item					4
Success Payments					$ 6,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| License Patent Rights
Upfront fee						$ 200
Stock issued to licensors \| shares						561,531
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Cpf1 License Agreement
License Agreement Contract Termination Term								4 months
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Cpf1 Market Cap Success Payments
Market capitalization	1,000,000							$ 1,000,000			$ 750,000
Notes payable payment terms								150 days
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Cas9-II License Agreement
Market capitalization	1,000,000							$ 1,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Maximum \| Cpf1 License Agreement
Research and development				$ 16,500
Royalties credit paid to third party (as a percent)								50.00%
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Maximum \| Cpf1 Success Payments
Success Payments								$ 10,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Maximum \| Cpf1 Market Cap Success Payments
Success Payments								125,000
Market capitalization threshold	10,000,000							10,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Maximum \| Cas9-II License Agreement
Success Payments								30,000
Market capitalization threshold	9,000,000							9,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Minimum \| Cpf1 Success Payments
Success Payments								750,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Minimum \| Cas9-II License Agreement
Market capitalization threshold	1,000,000							1,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Rare disease \| Maximum
Potential liability for future sales milestone payments						$ 36,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| Rare disease \| Maximum \| Cpf1 License Agreement
Potential liability for future sales milestone payments	36,000							36,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| United States Europe And Japan \| Maximum \| License Patent Rights
Potential liability for future clinical and regulatory milestone payments related to product approval						14,800
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| United States Europe And Japan \| Human disease \| Maximum \| Cpf1 License Agreement
Potential liability for future clinical and regulatory milestone payments related to product approval	20,000							20,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| United States Europe And Japan \| Human disease \| Maximum \| Cas9-II License Agreement
Potential liability for future sales milestone payments	13,500							13,500
Potential liability for future clinical and regulatory milestone payments related to product approval												$ 3,700
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| United States Europe And Japan \| Rare disease \| Maximum \| Cpf1 License Agreement
Potential liability for future clinical and regulatory milestone payments related to product approval	6,000							6,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| United States Europe And Japan \| Rare disease \| Maximum \| Cas9-II License Agreement
Potential liability for future sales milestone payments	9,000							9,000
Potential liability for future clinical and regulatory milestone payments related to product approval	1,100							1,100
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| United States and one other \| Human disease \| Maximum \| License Patent Rights
Potential liability for future clinical and regulatory milestone payments related to product approval						4,100
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| UNITED STATES \| Human disease \| Maximum \| License Patent Rights
Potential liability for future sales milestone payments						$ 54,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) \| UNITED STATES \| Human disease \| Maximum \| Cpf1 License Agreement
Potential liability for future sales milestone payments	54,000							$ 54,000
The Broad Institute Inc \| Cpf1 License Agreement
Stock issued to licensors \| shares		371,166	108,104
Success Payments	$ 7,500

Preferred Stock (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Feb. 08, 2016
Preferred Stock
Authorized common stock	195,000,000	195,000,000	195,000,000
Authorized preferred stock	5,000,000	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Common Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
Common Stock (Details) $ / shares in Units, $ in Thousands	Jan. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) Vote shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Mar. 31, 2017 USD ($)
Common Stock, Capital Shares Reserved for Future Issuance		7,621,134	6,309,915
Proceeds from public offering of common stock, net of issuance costs \| $		$ 154,143	$ 97,488
March 2017 ATM program \| Cowen and Company LLC (“Cowen)
Aggregate sales proceeds of stock \| $					$ 50,000
Commission payable (as a percent)					3.00%
Common Stock.
Voting rights per share \| Vote		1
Dividends, Common Stock \| $		$ 0	$ 0	$ 0
Number of common stock issued			6,785,000
Common Stock. \| Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance		225,000
2013 Plan \| Common Stock. \| Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance		1,220,567	1,595,082
2015 Plan \| Common Stock.
Common Stock, Capital Shares Reserved for Future Issuance		2,502,338	2,760,472
2015 Plan \| Common Stock. \| Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance		2,921,987	1,569,746
2015 Employee Stock Purchase Plan
Common Stock, Capital Shares Reserved for Future Issuance		751,242	384,615
Subsequent Event
Price per share \| $ / shares	$ 29.76
Subsequent Event \| March 2017 ATM program \| Cowen and Company LLC (“Cowen)
Number of common stock issued	1,429,205
Price per share \| $ / shares	$ 34.99
Proceeds from public offering of common stock, net of issuance costs \| $	$ 50,000

Stock Based Compensation - 2013 Stock Incentive Plan (Details)		1 Months Ended			12 Months Ended	52 Months Ended
	Jan. 01, 2018 shares	Jul. 31, 2015 shares	Apr. 30, 2015 shares	Jun. 30, 2014 shares	Dec. 31, 2017 $ / shares shares	Dec. 31, 2017 item shares	Dec. 31, 2016 shares	Sep. 30, 2013 shares
Stock-based compensation disclosures
Shares reserved for future awards					7,621,134	7,621,134	6,309,915
Employee and Consultant Options
Stock-based compensation disclosures
Options granted (in shares)					1,392,689
Restricted Stock
Stock-based compensation disclosures
Shares granted					480,000
Issued (in dollars per share) \| $ / shares					$ 28.05
2013 Plan
Stock-based compensation disclosures
Shares authorized								1,057,692
Increase to number of shares authorized		3,740,847	153,846	1,365,384
2013 Plan \| Outstanding stock options
Stock-based compensation disclosures
Vesting period					4 years
Expiration period					10 years
2013 Plan \| Performance based awards
Stock-based compensation disclosures
Number of awards with performance vesting criteria \| item						1
2015 Plan \| Subsequent Event
Stock-based compensation disclosures
Increase to number of shares authorized	1,801,017
2015 Employee Stock Purchase Plan
Stock-based compensation disclosures
Shares reserved for future awards					751,242	751,242	384,615
2015 Employee Stock Purchase Plan \| Subsequent Event
Stock-based compensation disclosures
Increase to number of shares authorized	450,254
Maximum \| 2013 Plan \| Consultant Stock Options
Stock-based compensation disclosures
Vesting period					4 years
Maximum \| 2015 Plan
Stock-based compensation disclosures
Shares reserved for future awards					2,923,076	2,923,076
Percent of shares outstanding reserved for future awards (as a percent)					4.00%
Maximum \| 2015 Employee Stock Purchase Plan
Stock-based compensation disclosures
Shares reserved for future awards					769,230	769,230
Percent of shares outstanding reserved for future awards (as a percent)					1.00%
Minimum \| 2013 Plan \| Consultant Stock Options
Stock-based compensation disclosures
Vesting period					1 year

Stock Based Compensation - Founder Awards (Details) - Founder Awards	1 Months Ended
	Jun. 30, 2014 USD ($) item shares	Sep. 30, 2013 shares
Stock-based compensation disclosures
Shares granted		2,403,845
Vesting percentage at issuance of Series A Preferred Stock		25.00%
Monthly vesting percentage after issuance of Series A Preferred Stock		1.5625%
Number of founders not providing service \| item	1
Shares repurchased	285,457
Payments for repurchase of shares \| $	$ 74

Stock-based Compensation - Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation disclosures
Compensation expense	$ 23,364	$ 16,881	$ 3,513
Research and development.
Stock-based compensation disclosures
Compensation expense	15,131	4,234	3,015
General and administrative
Stock-based compensation disclosures
Compensation expense	$ 8,233	$ 12,647	$ 498

Stock-based Compensation - Restricted Stock and Stock Options (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in unvested stock options
Compensation expense	$ 23,364,000	$ 16,881,000	$ 3,513,000
Restricted Stock
Changes in unvested restricted stock
Unvested restricted common stock, beginning of period (in shares)	822,638
Issued (in shares)	480,000
Vested (in shares)	(784,119)
Forfeited (in shares)	(5,294)
Unvested restricted common stock, end of period(in shares)	513,225	822,638
Weighted Average Grant Date Fair Value
Balance, beginning of period	$ 0.02
Issued (in dollars per share)	28.05
Vested (in dollars per share)	4.95
Forfeited (in dollars per share)	0.03
Balance, ending of period	$ 18.70	$ 0.02
Restricted Stock \| Employees
Changes in unvested stock options
Compensation expense	$ 500,000	$ 0	0
Unrecognized stock-based compensation expense	0
Restricted Stock \| Non-employees
Changes in unvested stock options
Compensation expense	4,100,000	$ 8,300,000	2,300,000
Unrecognized stock-based compensation expense	$ 10,300,000
Period for recognition	4 years 8 months 12 days
Employee and Consultant Options
Changes in unvested stock options
Outstanding, beginning of period (in shares)	3,411,783
Granted (in shares)	1,392,689
Exercised (in shares)	(289,583)			(75,304)
Cancelled (in shares)	(142,753)
Outstanding, end of period (in shares)	4,372,136	3,411,783
Vested and expected to vest (in shares)	4,372,126
Exercisable (in shares)	1,540,023
Outstanding, beginning of period (in dollars per share)	$ 13.71
Exercised (in dollars per share)				$ 0.03
Outstanding, end of period (in dollars per share)	17.28	$ 13.71
Vested and expected to vest outstanding, weighted average exercise price (in dollars per share)	17.28
Exercisable (in dollar per share)	$ 14.95
Remaining contractual life	8 years 6 months	8 years 9 months 18 days
Vested and expected to vest, remaining contractual life	8 years 6 months
Exercisable, remaining contractual life	8 years 3 months 18 days
Aggregate intrinsic value	$ 60,591,000	$ 16,190,000
Vesting and expected to vest, aggregate intrinsic value	60,591,000
Exercisable, aggregated intrinsic value	$ 25,099,000
Unvested stock options (in shares)	4,572	21,955
Intrinsic value of options exercised	$ 5,000,000	$ 900,000	$ 100,000
Weighted average fair value of options granted (per share)	$ 16.07	$ 14.10	$ 5.91
Employee and Consultant Options \| Employees and directors
Changes in unvested stock options
Compensation expense	$ 12,300,000	$ 6,000,000	$ 700,000
Employee and Consultant Options \| Employees
Changes in unvested stock options
Unrecognized stock-based compensation expense	$ 33,400,000
Period for recognition	2 years 6 months

Stock-based Compensation - Assumptions (Details) - Employee and Consultant Options - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employees and directors
Assumptions
Expected volatility	77.80%	78.40%	78.80%
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Risk free interest rate	2.10%	1.50%	1.70%
Employees
Assumptions
Unrecognized stock-based compensation expense	$ 33.4
Period for recognition	2 years 6 months
Non-employees
Assumptions
Expected volatility		76.50%	80.00%
Expected term (in years)		10 years	10 years
Risk free interest rate		1.60%	2.20%

401(K) Savings Plan (Details) - USD ($)	12 Months Ended
401(K) Savings Plan (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
401(k) Savings Plan
Employer match (as a percent)	200.00%
Maximum employee contributions eligible for matching contributions	$ 6,000
Maximum employee contributions eligible for matching contributions (as a percent)	3.00%
Contributions to the 401(k) Plan	$ 500,000	$ 0	$ 0

Income Taxes - Effective Income Tax Rate (Details)	12 Months Ended
Income Taxes - Effective Income Tax Rate (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of income tax rate
Income tax computed at federal statutory tax rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	5.90%	3.50%	2.50%
General business credit carryovers	2.50%	1.50%	0.80%
Non-deductible expenses	(2.10%)	(3.60%)	(17.90%)
Federal tax rate reduction	(24.70%)
Change in valuation allowance	(15.60%)	(35.40%)	(19.40%)

Income Taxes - Tax Reform (Details)	12 Months Ended
Income Taxes - Tax Reform (Details)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax Cuts and Jobs Act
U.S. Federal corporate tax rate (as a percent)		34.00%	34.00%	34.00%
Forecast
Tax Cuts and Jobs Act
U.S. Federal corporate tax rate (as a percent)	21.00%

Income Taxes - Components of Deferred Tax Assets And Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 27,726	$ 16,490
Tax credit carryforwards	5,259	2,014
Accrued expenses	2,079	7,353
Capitalized patent costs	26,307	16,025
Deferred revenue	7,151	9,672
Construction financing lease obligation	9,352	13,685
Other	4,978	2,979
Total deferred tax assets	82,852	68,218
Less valuation allowance	(73,301)	(54,300)
Net deferred tax assets	9,551	13,918
Deferred tax liabilities - depreciation and amortization	$ (9,551)	$ (13,918)

Income Taxes - Net Operating Losses (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Net operating losses	$ 202.7	$ 82.4
Research and development
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	$ 5.6	$ 2.3

Income Taxes - Other Narrative Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Section 382 period		3 years
Section 382 percentage		50.00%
Valuation allowance		$ 73,301	$ 54,300
Change in the valuation allowance		$ 19,000
U.S. Federal corporate tax rate (as a percent)		34.00%	34.00%	34.00%
Unrecognized tax benefits		$ 0	$ 0
Forecast
U.S. Federal corporate tax rate (as a percent)	21.00%

Net Loss per Share (Details) - shares	1 Months Ended			12 Months Ended
Net Loss per Share (Details) - shares	Dec. 31, 2017	Mar. 31, 2017	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2016
Potentially dilutive securities
Anti-dilutive common stock equivalent shares				5,130,247	4,234,421
Restricted Stock
Potentially dilutive securities
Anti-dilutive common stock equivalent shares				513,225	822,638
Outstanding stock options
Potentially dilutive securities
Anti-dilutive common stock equivalent shares				4,372,126	3,411,783
Convertible notes
Potentially dilutive securities
Anti-dilutive common stock equivalent shares				244,896
Initial Public Offering
Potentially dilutive securities
Number of common stock issued	2,265,500	4,600,000	6,785,000

Related-party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Rent and facility related fees
Related Party Transaction [Line Items]
Fees paid to related party	$ 0.0	$ 1.4	$ 1.2
Fees received from related party	$ 0.8	$ 0.0
Investor(s)
Related Party Transaction [Line Items]
Professional fees			$ 0.1

Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Data
Total revenue	$ 3,667	$ 6,282	$ 3,097	$ 682	$ 898	$ 962	$ 3,388	$ 805	$ 13,728	$ 6,053	$ 1,629
Total operating expenses	40,109	33,031	29,212	31,309	39,882	22,127	22,588	18,644	133,661	103,241	36,941
Total other income (expense), net	253	150	(324)	(470)	(392)	145	158	94	(391)	5	(37,588)
Net loss	(36,189)	(26,599)	(26,439)	(31,097)	(39,376)	(21,020)	(19,042)	(17,745)	(120,324)	(97,183)	(72,900)
Net loss applicable to common stockholders	$ (36,189)	$ (26,599)	$ (26,439)	$ (31,097)	$ (39,376)	$ (21,020)	$ (19,042)	$ (17,792)	$ (120,324)	$ (97,230)	$ (73,294)
Net loss per share applicable to common stockholders - basic and diluted	$ (0.84)	$ (0.64)	$ (0.65)	$ (0.85)	$ (1.10)	$ (0.59)	$ (0.54)	$ (0.80)

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Jan. 31, 2018	Sep. 30, 2017	Aug. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Gross proceeds from issuance of shares				$ 154,143	$ 97,488
Cpf1 License Agreement \| The Broad Institute Inc
Subsequent Event [Line Items]
Common stock issued		371,166	108,104
Subsequent Event
Subsequent Event [Line Items]
Payments for purchase of assets	$ 1,600
Number of shares issued under asset purchase agreement	56,099
Price per share	$ 29.76
Subsequent Event \| 2016 License Agreements \| The General Hospital (MGH)
Subsequent Event [Line Items]
Common stock issued	80,000
Subsequent Event \| March 2017 ATM program \| Cowen and Company LLC (“Cowen)
Subsequent Event [Line Items]
Number of common stock issued	1,429,205
Price per share	$ 34.99
Gross proceeds from issuance of shares	$ 50,000
Success Payment Notes \| Cpf1 License Agreement \| The Broad Institute Inc
Subsequent Event [Line Items]
Common stock issued			271,347
Success Payment Notes \| Subsequent Event \| Cpf1 License Agreement \| The Broad Institute Inc
Subsequent Event [Line Items]
Common stock issued	225,909