Editas Medicine (EDIT) 10-K 3 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2016	Feb. 01, 2017	Jun. 30, 2016
Document and Entity Information
Entity Registrant Name	Editas Medicine, Inc.
Entity Central Index Key	1,650,664
Document Type	10-K
Document Period End Date	Dec. 31, 2016
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 442,566,248
Entity Common Stock, Shares Outstanding		36,679,926
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 185,323	$ 143,180
Accounts receivable	88	1,019
Prepaid expenses and other current assets	1,772	786
Total current assets	187,183	144,985
Property and equipment, net	40,378	2,130
Restricted cash and other non-current assets	1,621	2,248
Total assets	229,182	149,363
Current liabilities:
Accounts payable	4,640	1,381
Accrued expenses	17,439	5,456
Notes payable	10,000
Deferred rent, current portion	18	88
Other current liabilities	986
Total current liabilities	33,083	6,925
Deferred rent, net of current portion	380
Deferred revenue, net of current portion	26,000	25,321
Warrant to purchase redeemable securities		289
Construction financing lease obligation, net of current portion	35,096
Other non-current liabilities	16	27
Total liabilities	94,575	32,562
Commitments and contingencies (see note 7)
Stockholders’ equity (deficit)
Preferred stock, $0.0001 par value per share: 5,000,000 shares and no shares authorized, at December 31, 2016 and December 31, 2015, respectively; no shares issued or outstanding at December 31, 2016 and December 31, 2015, respectively
Common stock, $0.0001 par value per share: 195,000,000 shares and 92,000,000 shares authorized at December 31, 2016 and December 31, 2015, respectively; 36,662,724 and 4,869,829 shares issued and 35,818,131 and 3,233,638 shares outstanding at December 31, 2016 and December 31, 2015, respectively	4
Additional paid-in capital	320,129	5,234
Accumulated deficit	(185,526)	(88,348)
Total stockholders’ equity (deficit)	134,607	(83,114)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 229,182	149,363
Series A-1 redeemable convertible
Current liabilities:
Redeemable convertible preferred stock		21,137
Series A-2 redeemable convertible
Current liabilities:
Redeemable convertible preferred stock		59,027
Series B redeemable convertible
Current liabilities:
Redeemable convertible preferred stock		$ 119,751

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	0
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	92,000,000
Common stock, shares issued	36,662,724	4,869,829
Common stock, shares outstanding	35,818,131	3,233,638
Series A-1 redeemable convertible
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	21,320,000
Redeemable convertible preferred stock, shares issued	0	21,260,000
Redeemable convertible preferred stock, shares outstanding	0	21,260,000
Series A-2 redeemable convertible
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	16,890,699
Redeemable convertible preferred stock, shares issued	0	16,890,699
Redeemable convertible preferred stock, shares outstanding	0	16,890,699
Series B redeemable convertible
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	26,666,660
Redeemable convertible preferred stock, shares issued	0	26,666,660
Redeemable convertible preferred stock, shares outstanding	0	26,666,660

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration and other research and development revenues	$ 6,053	$ 1,629
Operating expenses:
Research and development	56,979	18,846	$ 5,073
General and administrative	46,262	18,095	7,650
Total operating expenses	103,241	36,941	12,723
Operating loss	(97,188)	(35,312)	(12,723)
Other income (expense), net
Other expense, net	(57)	(37,445)	(928)
Interest income (expense), net	62	(143)	(34)
Total other income (expense), net	5	(37,588)	(962)
Net loss and comprehensive loss	(97,183)	(72,900)	(13,685)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(97,183)	(72,900)	(13,685)
Accretion of redeemable convertible preferred stock to redemption value	(47)	(394)	(309)
Net loss attributable to common stockholders	$ (97,230)	$ (73,294)	$ (13,994)
Net loss per share attributable to common stockholders, basic and diluted	$ (3.02)	$ (28.55)	$ (12.46)
Weighted-average common shares outstanding, basic and diluted	32,219,717	2,566,916	1,123,098

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Series A-1	Series A-2	Series B redeemable convertible	Common Stock.	Additional Paid-In Capital	Accumulated Deficit	Total
Balance, beginning of period at Dec. 31, 2013	$ 2,111
Balance, beginning of period (in shares) at Dec. 31, 2013	3,260,000
Statements of Redeemable Convertible Preferred Stock
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and issuance costs	$ 17,980
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and issuance costs (in shares)	18,000,000
Reclassification of tranche rights upon issuance of redeemable convertible preferred stock	$ 372						$ 372
Accretion of redeemable convertible preferred stock to redemption value	309						309
Balance, end of period at Dec. 31, 2014	$ 20,772
Balance, end of period (in shares) at Dec. 31, 2014	21,260,000
Balance, beginning of period at Dec. 31, 2013						$ (1,763)	(1,763)
Balance, beginning of period (in shares) at Dec. 31, 2013				751,200
Statement of Stockholders' Deficit
Issuance of common stock to licensors					$ 408		408
Issuance of common stock to licensors (in shares)				628,379
Vesting of restricted common stock and common stock subject to repurchase					2		2
Vesting of restricted common stock and common stock subject to repurchase (in shares)				77,942
Vesting of founders shares					48		48
Vesting of founders shares (in shares)				405,648
Accretion of redeemable convertible preferred stock to redemption value					(309)		(309)
Stock-based compensation expense					7		7
Net loss						(13,685)	(13,685)
Balance, end of period at Dec. 31, 2014					156	(15,448)	(15,292)
Balance, end of period (in shares) at Dec. 31, 2014				1,863,169
Statements of Redeemable Convertible Preferred Stock
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and issuance costs		$ 21,989	$ 119,722
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and 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	$ 365		$ 29				394
Balance, end of period at Dec. 31, 2015	$ 21,137	$ 59,027	$ 119,751
Balance, end of period (in shares) at Dec. 31, 2015	21,260,000	16,890,699	26,666,660
Statement of Stockholders' Deficit
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
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
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	$ 40		$ 7				47
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering	$ (21,177)	$ (59,027)	$ (119,758)
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering (in shares)	(21,260,000)	(16,890,699)	(26,666,660)
Balance, end of period (in shares) at Dec. 31, 2016			0
Statement of Stockholders' Deficit
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
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 initial public offering, net of issuance costs				$ 1	97,487		97,488
Issuance of common stock from initial 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
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

Consolidated Statements of Red6

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock issuance costs		$ 1,746
Common Stock.
Stock issuance costs	$ 11,100
Series A-1
Stock issuance costs			$ 20
Series A-2
Stock issuance costs		1
Series B redeemable convertible
Stock issuance costs		$ 278

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flow from operating activities
Net loss	$ (97,183)	$ (72,900)	$ (13,685)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	16,881	3,513	55
Depreciation	1,202	471	157
Changes in fair value of warrant liability	87
Interest related to facility lease obligation	445
Non-cash research and development expenses	10,000		730
Re-measurement of warrant to purchase redeemable securities		241	(2)
Change in fair value of preferred stock tranche asset or liability		35,551	938
Changes in fair value of anti-dilutive protection liability		1,609	5
Changes in deferred rent	310	(96)	184
Other non-cash items, net	114	149	19
Changes in operating assets and liabilities:
Accounts receivable	931	(1,019)
Prepaid expenses and other current assets	(986)	(373)	(88)
Other non-current assets	2,246	40	(20)
Accounts payable	3,251	(1,436)	2,191
Accrued expenses	11,841	3,526	861
Deferred revenue	935	25,321
Net cash used in operating activities	(49,926)	(5,403)	(8,655)
Cash flow from investing activities
Purchases of property and equipment	(3,493)	(1,431)	(1,217)
Proceeds from the sale of equipment	20
Changes in restricted cash	(1,619)
Net cash used in investing activities	(5,092)	(1,431)	(1,217)
Cash flow from financing activities
Proceeds from equipment loan, net of issuance costs		1,500	462
Proceeds from initial public offering of common stock, net of issuance costs	97,488
Proceeds from exercise of stock options	233
Payments on construction financing lease obligation	(560)
Proceeds from the issuance of redeemable convertible preferred stock and tranche rights, net of issuance costs		141,711	17,980
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	41
Payments of initial public offering costs		(1,746)
Net cash provided by financing activities	97,161	139,391	18,483
Net increase in cash and cash equivalents	42,143	132,557	8,611
Cash and cash equivalents, beginning of period	143,180	10,623	2,012
Cash and cash equivalents, end of period	185,323	143,180	10,623
Supplemental disclosure of cash and non-cash activities:
Accretion of redeemable convertible preferred stock to redemption value	47	394	309
Fixed asset additions included in accounts payable and accrued expenses	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	17	2
Accrual of final payment fee on equipment loan and debt discount			20
Interest paid	$ 465	91
Initial public offering costs incurred but unpaid at period end		502
Reclassification of tranche rights upon issuance of redeemable convertible preferred stock		$ 37,038	$ 372

Nature of Business

Nature of Business	12 Months Ended
	Dec. 31, 2016
Nature of Business
Nature of Business	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, and has financed its operations through various equity and debt financings, including the initial public offering of its common stock (“IPO”), private placements of preferred stock and an equipment loan, and from upfront, milestone and research and development fees paid under a research collaboration with Juno Therapeutics, Inc. (“Juno Therapeutics”), and from research and development payments from the Cystic Fibrosis Foundation Therapeutics, Inc . (“CFFT”), a non-profit drug discovery and development affiliate of the Cystic Fibrosis Foundation. 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 overallotment 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 connection with the IPO, the board of directors and the stockholders of the Company approved a one-for-2.6 reverse stock split of the Company’s issued and outstanding common stock. The reverse stock split became effective on January 15, 2016. All share and per share amounts in the consolidated financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split, including reclassifying an amount equal to the reduction in par value to additional paid-in capital. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 24,929,709 shares of common stock. As of December 31, 2016, there were 35,818,131 shares of common stock outstanding. The significant increase in shares outstanding in the first quarter of 2016 is expected to impact the year-over-year comparability of the Company’s net loss per share calculations for the next three months. The Company has incurred annual net operating losses in every year since its inception. The Company expects that its existing cash and cash equivalents at December 31, 2016, anticipated interest income, anticipated research support under the Company’s collaboration agreement with Juno Therapeutics, anticipated payments from CFFT, and anticipated payments from the Company’s subtenant will enable it to fund its operating expenses and capital expenditure requirements for at least the next 18 months following the date of this Annual Report on Form 10-K. The Company will require substantial additional capital to fund operations beyond this date. The Company has not generated any product revenues and has financed its operations primarily through public offerings and private placements of its equity securities, and funding from its collaboration with Juno Therapeutics. 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 accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2016
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”). 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, valuation of the redeemable convertible preferred stock tranche liability and the anti-dilutive protection liability, valuation of the warrant liability, deferred tax valuation allowances, the fair value of common stock prior to the completion of the IPO, and the construction lease financing obligation. 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, or exit 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, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short‑term nature. Cash and Cash Equivalents 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. Restricted Cash The Company had restricted cash of $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, 2016 and 2015, respectively. The restricted cash balance is included within restricted cash and other non-current assets and other current assets in the accompanying consolidated balance sheets at December 31, 2016 and 2015, respectively. 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. 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, 2016 and 2015. 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 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, 2016. Revenue Recognition To date, the Company has earned revenue under the collaboration and license agreement with Juno Therapeutics and its award agreement with CFFT (see Note 8). 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, consultant fees, facility costs including rent, depreciation, and maintenance expenses, and fees for acquiring and maintaining licenses under third party licensing agreements. 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. Warrants to Purchase Convertible Preferred Stock In conjunction with an equipment loan financing, the Company issued to Silicon Valley Bank a warrant to purchase up to 60,000 shares of the Company’s Series A‑1 preferred stock at an exercise price of $1.00 per share. The fair value of the warrant at the issuance date was recorded as a reduction to face value of the debt balance and was amortized as interest expense, along with other debt issuance costs, over the term of the loan. Due to the liquidation preferences of the Series A‑1 preferred stock, the Company recorded the warrant as a liability on the consolidated balance sheets. Following a reverse stock split in January 2016 and the closing of the IPO in February 2016, such warrant converted into a warrant to purchase 23,076 shares of common stock, which Silicon Valley Bank fully exercised in February 2016 pursuant to a net exercise provision for an aggregate of 19,271 shares of common stock. 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. B ecause there had been no public market for the Company’s common stock prior to the IPO, there is 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 . Audit and Accounting Practice Aid Series: Valuation of Privately Held Company Equity Securities Issued as Compensation 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 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. Prior to 2016, other income (expense), net included re‑measurement gains or losses associated with changes in the fair value of the tranche rights associated with the Series A‑1 preferred stock and anti‑dilutive protection liability associated with the issuance of common stock to certain licensors. In June 2015, upon the issuance of the final tranche of Series A preferred stock, the tranche right liability was settled and reclassified to Series A preferred stock and the anti‑dilutive protection liability was settled and reclassified to additional paid‑in‑capital. Therefore no further re‑measurement gains or losses will be recognized related to the tranche rights or the anti‑dilutive protection liability. 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. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements. 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 and cash equivalents and accounts receivable. The Company’s cash and cash equivalents 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 (Note 8) for which the Company does not obtain collateral. As of December 31, 2016, all of the Company’s revenue to date had been generated exclusively from the collaboration with Juno Therapeutics and its award agreement with CFFT. 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. Subsequent Events The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. 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, 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 Company 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. Early adoption is permitted beginning after December 15, 2016, including interim reporting periods within those years. 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 two revenue arrangements, its license and collaboration with Juno Therapeutics and its arrangement with CFFT pursuant to which it has recognized a total of $5.7 million and $0.3 million, respectively, through December 31, 2016. 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 two arrangements. This analysis includes, but is not limited to, reviewing variable consideration as it relates to its agreements, reviewing the method and timing of recognition of the license payment, research funding and the $2.5 million milestone received from Juno Therapeutics, assessing potential disclosures and evaluating the impact of each potential method of adoption on the Company’s consolidated financial statements. In addition, the Company continues to monitor additional changes, modifications, clarifications or interpretations undertaken by the FASB, which may impact its conclusions. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements - Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (“ASU 2014-15”). ASU 2014-15 is intended to define management’s responsibility to evaluate whether there is substantial doubt about an organization’s ability to continue as a going concern and to provide related footnote disclosures, if required. ASU 2014-15 is effective for annual reporting periods ending after December 15, 2016, and applies to annual and interim periods thereafter. The Company adopted this standard during the three months ended December 31, 2016. The Company completed its evaluation and determined there was not substantial doubt about the organization’s ability to continue as a going concern. 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 impact that this ASU may have on its consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting , which simplifies share-based payment accounting through a variety of amendments. The standard will be effective for annual reporting periods and interim periods within those annual periods, beginning after December 15, 2016, and early adoption is permitted. The new standard became effective for the Company on January 1, 2017 and is not expected to have a material impact on the consolidated financial statements. In October 2016, the FASB issued ASU 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 is effective for fiscal years beginning after December 15, 2017, and interim periods within those years. Early adoption is permitted. The Company is evaluating the potential impact that the adoption of ASU 2016-18 will have on the Company’s financial position or results of operations.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2016
Fair Value Measurements
Fair Value Measurements	3. Fair Value Measurements Assets 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 $ $ — $ — Money market funds, included in restricted cash and other non-current assets — — Total financial assets $ $ $ — $ — Assets and liabilities measured at fair value on a recurring basis as of December 31, 2015 are as follows (in thousands): Quoted Prices Significant in Active Other Significant Markets for Observable Unobservable December 31, Identical Assets Inputs Inputs Financial Assets 2015 (Level 1) (Level 2) (Level 3) Cash and cash equivalents $ $ $ — $ — Money market funds, included in other current assets — — Total financial assets $ $ $ — $ — Financial Liabilities Warrant liability $ $ — $ — $ Total financial liabilities $ $ — $ — $ There were no transfers between fair value measurement levels during the years ended December 31, 2016 or 2015. The fair value of the preferred stock warrant liability was determined based on “Level 3” inputs utilizing the Black-Scholes option pricing model. Upon the completion of the IPO, the Company’s 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. The following table provides a roll‑forward of the fair value of the assets and liabilities measured at fair value on a recurring basis using Level 3 significant unobservable inputs (in thousands): Warrant Liability Balance at December 31, 2015 $ Changes in fair value, included in other income (expense), net Reclassification to additional paid-in capital in connection with the initial public offering Balance at December 31, 2016 $ —

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2016
Prepaid Expenses and Other Current Assets
Prepaid Expenses and Other Current Assets	4. Prepaid Expenses and Other Current Assets Prepaid expense and other current assets consisted of the following (in thousands): As of December 31, 2016 2015 Prepaid expenses $ $ Restricted cash — Other Total $ $

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2016
Property and Equipment, Net
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): As of December 31, 2016 2015 Building $ $ — Laboratory equipment Computer equipment Furniture and office equipment Leasehold improvements Software — Total property and equipment Less: accumulated depreciation Property and equipment, net $ $ The Company recorded $1.2 million, $0.5 million, and $0.2 million in depreciation expense during the years ended December 31, 2016, 2015 and 2014, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2016
Accrued Expenses
Accrued Expenses	6. Accrued Expenses Accrued expenses consisted of the following (in thousands): As of December 31, 2016 2015 Patent and license fees $ $ Deferred initial public offering costs — Employee compensation costs Professional services Research and development — Other Total $ $

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies.
Commitments and Contingencies	7. Commitments and Contingencies Facility Leases In December 2013, the Company entered into an agreement to sublease its facility under a non-cancelable operating lease that was set to expire at the end of September 2016. The Company extended the sublease through November 1, 2016. Pursuant to the sublease agreement, the Company maintained restricted cash of $0.3 million in a collateral account that was held until the termination of the Company’s obligations under the agreement. The sublease agreement could not be extended beyond the expiration date of the sublease. The sublease contained escalating rent clauses which required higher rent payments in future years. The Company expensed rent on a straight-line basis over the term of the sublease, including any rent-free periods. The deposit was recorded in prepaid expenses and other current assets in the accompanying consolidated balance sheet as of December 31, 2015. In November 2015, the Company entered into a real estate license agreement to sublease from the licensor additional laboratory space in Cambridge, Massachusetts. The term of the lease was from December 1, 2015 to November 30, 2016. The Company’s contractual obligation related to lease payments over the term of the sublease was approximately $1.9 million. Hurley Street, Cambridge, MA In February 2016, the Company entered into a lease agreement for approximately 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, 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. During the year ended December 31, 2016, the Company recognized $0.5 million of rental expense attributable to the land. 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 2017 $ 2018 2019 2020 2021 2022 and thereafter Total minimum lease payments $ Rent expense of approximately $2.5 million, $1.0 million, and $0.9 million was incurred during the years ended December 31, 2016, 2015 and 2014, respectively. Licensor Expense Reimbursement The Company is obligated to reimburse The Broad Institute Inc. (the “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 $23.1 million, $9.4 million and $1.7 million in expense during the years ended December 31, 2016, 2015 and 2014, respectively, for such reimbursement. Success Payments In 2016, the Company entered into patent license agreements with 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, into common stock of the Company subject to certain conditions. The Success Payments are payable, if and when, the Company’s market capitalization reaches specified thresholds or upon a sale of the Company for consideration in excess of those thresholds, as discussed more fully in Note 8 (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 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 Company did not incur any expenses related to Success Payments during the year ended December 31, 2016. Notes Payable In December 2016, in connection with the Company’s entry into certain of the 2016 License Agreements with Broad, it issued promissory notes (the “Notes”), in an aggregate principal amount of $10.0 million to Broad and Wageningen. Principal and interest on the Notes is payable in December 2017, or if earlier, a specified period of time following a company sale event. The Notes bear interest at a rate of 4.8% per annum. The Company may elect to make any payment of amounts outstanding under the Notes either in the form of cash or, subject to certain conditions, in shares of the Company’s common stock of equal value, with such shares being valued for such purpose at the closing price of the Company’s common stock as reported the NASDAQ Stock Market for the trading day immediately preceding the date of such payment if the Company’s common stock is then listed on the NASDAQ Stock Market. In the event of a change of control of the Company or a sale of the Company, the Company will be required to pay all remaining principal and accrued interest on the Notes in cash within a specified period following such event. Under the terms of certain of the 2016 License Agreements with Broad, the Company may be required to issue additional promissory notes in connection with potential Success Payments. The Company believes that the carrying value of the 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, 2016 and 2015.

Significant Agreements

Significant Agreements	12 Months Ended
	Dec. 31, 2016
Significant Agreements
Significant Agreements	8. 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 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 IND 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 (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 (the “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 will pay to the Company an aggregate of up to $22.0 million in research and development funding over the initial five year term of the research program 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 five year term of the research program 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 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. In May 2016, the Company achieved a $2.5 million milestone under the collaboration resulting from technical progress in a research program to create engineered T cells with CARs and TCRs to treat cancer. 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, 2016, the next potential milestone payment that the Company may be entitled to receive under the 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 Revenue Recognition—Multiple Element Arrangements . 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 VSOE of selling price nor TPE 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, 2016 and 2015, the Company recognized revenue totaling approximately $5.7 million ($2.5 million of which related to the first milestone payment) and $1.6 million, respectively, with respect to the collaboration with Juno Therapeutics. The Company did not recognize any collaboration revenue in the year ended December 31, 2014. The revenue is classified as collaboration and other research and development revenue in the accompanying consolidated statement of operations. As of December 31, 2016, there was approximately $26.0 million of deferred revenue related to the Company’s collaboration with Juno Therapeutics, which is classified as long-term on the consolidated balance sheet. As of December 31, 2015, there was approximately $25.3 million of deferred revenue related to the Company’s collaboration with Juno Therapeutics, which was classified as long-term in the accompanying consolidated balance sheet. Cystic Fibrosis Foundation Therapeutics, Inc. Award Agreement In May 2016, the Company entered into an award agreement (the “CF Award Agreement”) with CFFT, pursuant to which the Company received a development award for up to $5.0 million in funding over the agreement’s three year term (the “Award”). The funding from the Award is supporting the Company’s cystic fibrosis development program and related technology research and development. The Company is required to contribute additional funds to the program in an amount equal to the funds contributed by CFFT under the agreement. Pursuant to the terms of the CF Award Agreement, the Company is obligated to make royalty payments to CFFT contingent upon commercialization of an editing package, a delivery package, or a combination thereof, for modification of the cystic fibrosis transmembrane conductance regulator gene, the research or development of which was derived in whole or in part from the development program (a “CF Product”), including payments each equal to two times the amount the Company receives under the agreement, following the first commercial sale of a CF Product in the United States and the European Union, respectively. The Company is also obligated to make a payment to CFFT equal to two times the amount the Company receives under the CF Award Agreement, due in the first calendar year in which the aggregate cumulative net sales of a CF Product exceed $100.0 million. The payments due will not, in the aggregate, exceed ten percent of net sales of a CF Product in a year; the remaining obligation will be carried forward to subsequent year(s) until the payment of any such remaining payment does not, in the aggregate, exceed ten percent of net sales of a CF Product. The Company is also obligated to make payments to CFFT of up to two times the Award amount if the Company transfers, sells or licenses the development program technology, or if the Company enters into a change of control transaction, with such payments to be credited against the payments due upon commercialization. Following the first year anniversary of the effective date of the agreement, either party can terminate the agreement without cause by providing 90 days’ notice. The Company’s payment obligations survive the termination of the CF Award Agreement. During the year ended December 31, 2016, the Company recognized revenue of $0.3 million with respect to the Award. The revenue is classified as collaboration and other research and development revenue in the accompanying consolidated statement of operations. Adverum Biotechnologies, Inc. Collaboration, Option, and License Agreement In August 2016, the Company entered into an agreement with Adverum Biotechnologies, Inc. (“Adverum”) to explore the delivery of genome editing medicines to treat up to five inherited retinal diseases. Under the terms of the agreement, the Company paid an upfront non-refundable fee of $1.0 million to evaluate Adverum’s next generation adeno-associated viral vectors (“AAVs”) for use in clinical development. The Company will support all preclinical activities related to this agreement, including research and development activities to be performed by Adverum, with $0.5 million of the upfront fee being creditable against this funding obligation. Accordingly, the Company has deferred and capitalized $0.5 million of the $1.0 million upfront fee as an advance payment for future research and development activities which the Company believes will be incurred in the future. The capitalized amount will be expensed as research and development expenses in the Company’s consolidated statements of operations as the related services are performed. The Company expensed the remaining $0.5 million as research and development expense in the accompanying statement of operations during the year ended December 31, 2016. Additionally, the Company may pay, at its discretion, an additional fee of $1.0 million, per exercise, to exercise an option to receive an exclusive license to Adverum’s next generation AAVs for use in an indication chosen under the agreement. Adverum is also entitled to receive development and regulatory milestone payments up to a maximum of a mid-single digit millions of dollars per license based on the achievement of specific events for a product candidate that includes an Adverum vector (an “Adverum Product”) and a low to mid-single digit millions of dollars based on the achievement of specific events for a product candidate that does not include an Adverum vector (a “Non-Adverum Product”). Adverum is also entitled to receive certain commercial milestone payments for Adverum Products up to a maximum amount of a low double digit million dollar amount per product. The Company is also obligated to pay Adverum single digit to low double digit percentage royalties on net sales of Adverum Products and low single digit percentage royalties on sales of Non-Adverum Products sold in applicable territories during the royalty term. 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 (e.g. anti‑dilution protection liability), which was settled in June 2015. MGH is entitled to nominal annual license fees and to receive future clinical, regulatory and commercial milestone payments aggregating to a maximum of $3.7 million and aggregate 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 rec

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2016
Preferred Stock
Preferred Stock	9. Preferred Stock Redeemable Convertible Preferred Stock In November 2013, the Company entered into a preferred stock purchase agreement (the “Preferred Stock Agreement”) in which it agreed to sell, and the purchasers agreed to purchase up to $43 million of Series A ‑ 1 redeemable convertible preferred stock (“Series A ‑ 1 Preferred Stock”) and Series A ‑ 2 redeemable convertible preferred stock (“Series A ‑ 2 Preferred Stock” which together with the Series A ‑ 1 Preferred Stock is collectively referred to as “Series A Preferred Stock”) in three anticipated tranches. In November 2013, the Company sold 3,260,000 shares of Series A‑1 Preferred Stock in exchange for gross cash proceeds of $3.3 million. The Company issued 2,000,000 shares of Series A‑1 Preferred Stock in exchange for cash proceeds of $2.0 million, 2,500,000 shares of Series A‑1 Preferred Stock in exchange for cash proceeds of $2.5 million, and 500,000 shares of Series A‑1 Preferred Stock in exchange for cash proceeds of $500,000 in interim closings in May 2014, July 2014, and October 2014, respectively. In November 2014, the Company issued 13,000,000 shares of Series A‑1 Preferred Stock in exchange for cash proceeds of $13.0 million. Finally, the Company issued 16,698,672 shares of Series A‑2 Convertible Preferred Stock in exchange for cash proceeds of $21.7 million in June 2015. An executive of the Company purchased 192,027 shares of Series A‑2 Preferred Stock for $0.3 million. In August 2015, the Company sold 26,666,660 shares of Series B redeemable convertible preferred stock (“Series B Preferred Stock”) for cash proceeds of $120.0 million. Prior to the IPO, the holders of the Company's convertible Preferred Stock had certain voting, dividend rights, as well as liquidation preferences and conversion privileges. All rights, preferences, and privileges associated with the convertible Preferred Stock were terminated at the time of the Company's IPO in conjunction with the conversion of all outstanding shares of convertible Preferred Stock into shares of Common Stock. Upon the closing of the Company's IPO, all outstanding shares of the Company's Preferred Stock were automatically converted into 24,929,709 shares of Common Stock. Tranche Rights Issued with Series A Preferred Stock Included in the terms of the Preferred Stock Agreement were certain rights (“Tranche Rights”) granted to the purchasers of Series A‑1 Preferred Stock to purchase and the Company to sell additional shares of Series A‑1 and Series A‑2 Preferred Stock upon the achievement of performance milestones. The Company concluded the Tranche Rights met the definition of a freestanding financial instrument, as the Tranche Rights were legally detachable and separately exercisable from the Series A‑1 Preferred Stock. As the Series A Preferred Stock was redeemable at the election of holders of the then‑outstanding shares of Series A Preferred Stock, the Tranche Rights were classified as an asset or liability under ASC Topic 480, Distinguishing Liabilities from Equity , and were initially recorded at fair value. The Tranche Rights were then remeasured at fair value at each subsequent reporting period. The fair value of the portion of the Tranche Right, based on the implied value of the Preferred Stock from the Company’s third party valuation that was settled at each closing was reclassified to Series A ‑ 1 Preferred Stock. The Company recognized other expense of $35.5 million and $1.0 million related to the mark to market of Tranche Rights during the year ended December 31, 2015 and 2014, respectively. Preferred Stock On February 8, 2016, the Company filed a restated certificate of incorporation with the Secretary of State of the State of Delaware. The restated certificate amended and restated the Company’s certificate of incorporation in its entirety to, among other things increase the authorized number of shares of common stock to 195,000,000 shares, eliminate all references to the previously existing series of preferred stock, and authorize 5,000,000 shares of undesignated preferred stock that may be issued from time to time by the Company’s board of directors in one or more series. As of December 31, 2016, the Company had no shares of preferred stock issued or outstanding.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2016
Common Stock
Common Stock	10. 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, 2016: 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, 2016 2015 Shares reserved for redeemable convertible preferred stock outstanding — Shares reserved for future issuances of redeemable convertible preferred stock warrants — Shares reserved for outstanding stock option awards under the 2013 Stock Incentive Plan, as amended Remaining shares reserved, but unissued, for future awards under the 2013 Stock Incentive Plan, as amended — Shares reserved for outstanding stock option awards under the 2015 Stock Incentive Plan — Remaining shares reserved, but unissued, for future awards under the 2015 Stock Incentive Plan — Remaining shares reserved, but unissued, for future awards under the 2015 Employee Stock Purchase Plan —

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2016
Stock-Based Compensation
Stock-Based Compensation	11. 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. Awards granted to non‑employee consultants generally vest monthly over a period of one to four years. In connection with the Company’s IPO, the Company’s board of directors determined to grant no further awards under the 2013 Plan. As of December 31, 2016, there were 1,595,082 shares reserved for issuance upon the exercise of awards outstanding 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 Company’s 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. As of December 31, 2016, there were 1,569,746 shares reserved for issuance upon the exercise of awards outstanding under the 2015 Plan, and an additional 2,760,472 shares reserved for future 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. 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 Company’s IPO. As of December 31, 2016, there were 384,615 shares reserved for issuance under the 2015 ESPP. 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 Company has not issued any shares under the 2015 ESPP as of December 31, 2016. Founder Awards In September 2013, the Company issued 2,403,845 shares of restricted stock to its non‑employee founders for services rendered. 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 are subject to repurchase rights. Accordingly, the Company has recorded the proceeds from the issuance of restricted stock as a liability in its consolidated balance sheets. The restricted stock liability is reclassified into stockholders’ equity (deficit) as the restricted stock vests. In the event that a founder is no longer in the Company’s service (whether as a consultant, employee, director, or advisor) prior to the fourth anniversary of the vesting commencement date, the Company has the right to repurchase the unvested shares at $0.0003 per share. 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. Upon a change in control, all unvested founder shares will be released from the Company’s repurchase options. Stock‑based compensation expense associated with these awards is recognized as the awards vest. Unvested awards are 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 and comprehensive loss was as follows (in thousands): Year Ended December 31, 2016 2015 2014 Research and development $ $ $ General and administrative — Total stock-compensation expense $ $ $ Restricted Stock From time to time, upon approval by the 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 included as a component of other long term liabilities based on the scheduled vesting dates. 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, 2015 and 2016 is as follows: Weighted Average Grant Date Fair Value Shares Per Share Unvested Restricted Common Stock as of December 31, 2015 $ Issued — — Vested $ Unvested Restricted Common Stock as of December 31, 2016 $ 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, 2016, the Company had no unrecognized stock‑based compensation expense related to its employee unvested restricted stock awards. As of December 31, 2016, the Company had unrecognized stock‑based compensation expense related to its non‑employee unvested restricted stock awards of $3.4 million which is expected to be recognized over a remaining weighted average vesting period of 0.6 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 as a component of other long term liabilities based on the scheduled vesting dates. 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, 2016: Weighted Average Remaining Aggregate Intrinsic Shares Exercise Price Contractual Life Value (in thousands) Outstanding at December 31, 2015 $ $ Granted $ — — Exercised $ — — Cancelled $ — — Outstanding at December 31, 2016 $ $ Vested and expected to vest at December 31, 2016 $ $ Exercisable at December 31, 2016 $ $ The table above reflects unvested stock options as exercised on the dates that the shares are no longer subject to repurchase. The Company had 21,955 and 39,338 shares of unvested common stock at December 31, 2016 and 2015 related to the exercise of unvested stock options. The total intrinsic value of options exercised for the years ended December 31, 2016, 2015, and 2014 was $0.9 million, $0.1 million, and $0.0, respectively. The total fair value of employee options vested from the years ended December 31, 2016, 2015, and 2014 was $2.8 million, $8 thousand, and $0.0 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, 2016, 2015 and 2014 was $14.10, $5.91, and $0.03, respectively. The expense related to options granted to employees was $6.0 million, $0.7 million and $0.0 million for the years ended December 31, 2016, 2015 and 2014, respectively. There were no stock options granted during the period from September 3, 2013 to December 31, 2013. 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, 2016 2015 2014 Expected volatility % % % Expected term (in years) Risk free interest rate % % % Expected dividend yield — — — There were 100,000 options granted to persons other than employees and directors during the year ended December 31, 2016. For the year ended December 31, 2016, 2015 and 2014, 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, 2016 2015 2014 Expected volatility % % % Expected term (in years) Risk free interest rate % % % Expected dividend yield — — — As of December 31, 2016, the Company had unrecognized stock‑based compensation expense related to its employee stock options of $24.5 million which the Company expects to recognize over a remaining weighted average vesting period of 3.0 years.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2016
401(k) Savings Plan
401(k) Savings Plan	12. 401(k) Savings Plan The Company has a defined‑contribution savings plan under Section 401(k) of the Internal Revenue Code (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 did not make any contributions to the 401(k) Plan through December 31, 2016.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Taxes
Income Taxes	13. 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, 2016 2015 Income tax computed at federal statutory tax rate % % State taxes, net of federal benefit % % General business credit carryovers % % Non-deductible expenses % % Change in valuation allowance % % — % — % The principal components of the Company’s deferred tax assets and liabilities consist of the following at December 31, 2016 and 2015 (in thousands): Year Ended December 31, 2016 2015 Deferred tax assets: Net operating loss carryforwards $ $ Tax credit carryforwards Accrued expenses Capitalized patent costs Deferred revenue — Construction financing lease obligation — Other Total deferred tax assets Less valuation allowance Net deferred tax assets Deferred tax liabilities—depreciation and amortization Net deferred taxes $ — $ — The Company has incurred net operating losses (“NOL”) since inception. At December 31, 2016 and 2015, the Company had federal and state net operating loss carryforwards of $82.4 million (which include $0.7 million of NOL carryforwards from stock-based compensation) and $58.3 million (which include $0.1 million of NOL carryforwards from stock-based compensation), respectively, which expire beginning in 2033 and will continue to expire through 2036. As of December 31, 2016 and 2015, the Company had federal and state research and development tax credits carryforwards of $2.3 million and $0.8 million, respectively, which expire beginning in 2028 and will continue to expire through 2036. The Company has generated NOL carryforwards from stock-based compensation deductions in excess of expenses recognized for financial reporting purposes (“excess tax benefits”). Excess tax benefits are realized when they reduce taxes payable, as determined using a “with and without” method, and are credited to additional paid-in capital rather than as a reduction of the income tax provision. 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 and research and development credit carryforwards. 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 $54.3 million and $19.9 million has been established at December 31, 2016 and 2015, respectively. The change in the valuation allowance of $34.4 million for the year ended December 31, 2016 was primary due to additional operating losses, capitalized patent costs and deferred revenue. 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 take 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, 2016 and 2015, 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, 2016. There are no federal or state audits in process.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2016
Net Loss Per Share
Net Loss Per Share	14. 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 and warrants 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. Contingently issuable shares of common stock pursuant to the 2016 License Agreements and Notes (Note 7) are excluded from the calculation of basic and diluted net loss per share calculation as the Payment Conditions have not been satisfied. Upon the closing of the IPO in February 2016, the Company sold 6,785,000 shares of common stock and issued an additional 24,929,709 shares of common stock in connection with the automatic conversion of its redeemable convertible preferred stock. The issuance of these shares resulted in a significant increase in the Company’s weighted-average shares outstanding for the fiscal year ended December 31, 2016 when compared to the prior year 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 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, 2016 2015 Redeemable convertible preferred stock — Warrant to purchase redeemable convertible preferred stock — Unvested restricted common stock Outstanding stock options Total

Related-Party Transactions

Related-Party Transactions	12 Months Ended
	Dec. 31, 2016
Related-Party Transactions
Related-Party Transactions	15. Related‑Party Transactions During the year ended December 31, 2016 and 2015, the Company paid a related party $1.4 million and $1.2 million in rent and facility-related fees, respectively. In addition, during the year ended December 31, 2015 and 2014, the Company paid one of its investors $0.1 million and $0.2 million in professional fees, respectively.

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2016
Selected Quarterly Financial Data
Selected Quarterly Financial Data	16. Selected Quarterly Financial Data (unaudited) – The following table contains selected quarterly financial information from 2016 and 2015. 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, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (in thousands, except per share data) Total revenue $ $ $ $ Total operating expenses Total other expense, net Net loss $ $ $ $ Net loss applicable to common stockholders $ $ $ $ Net loss per share applicable to common stockholders — basic and diluted $ $ $ $ Three months ended March 31, 2015 June 30, 2015 September 30, 2015 December 31, 2015 (in thousands, except per share data) Total revenue $ — $ $ $ Total operating expenses Total other expense, net Net loss $ $ $ $ Net loss applicable to common stockholders $ $ $ $ Net loss per share applicable to common stockholders — basic and diluted $ $ $ $

Summary of significant accoun24

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2016
Summary of significant accounting policies
Principles of Consolidation	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	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”).
Use of Estimates	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, valuation of the redeemable convertible preferred stock tranche liability and the anti-dilutive protection liability, valuation of the warrant liability, deferred tax valuation allowances, the fair value of common stock prior to the completion of the IPO, and the construction lease financing obligation. 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	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, or exit 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, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short‑term nature.
Cash and Cash Equivalents	Cash and Cash Equivalents 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.
Restricted Cash	Restricted Cash The Company had restricted cash of $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, 2016 and 2015, respectively. The restricted cash balance is included within restricted cash and other non-current assets and other current assets in the accompanying consolidated balance sheets at December 31, 2016 and 2015, respectively.
Accounts Receivable	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. 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, 2016 and 2015.
Property and Equipment	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
Impairment of Long-lived Assets	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, 2016.
Revenue Recognition	Revenue Recognition To date, the Company has earned revenue under the collaboration and license agreement with Juno Therapeutics and its award agreement with CFFT (see Note 8). 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 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, consultant fees, facility costs including rent, depreciation, and maintenance expenses, and fees for acquiring and maintaining licenses under third party licensing agreements. 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	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	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.
Warrants to Purchase Convertible Preferred Stock	Warrants to Purchase Convertible Preferred Stock In conjunction with an equipment loan financing, the Company issued to Silicon Valley Bank a warrant to purchase up to 60,000 shares of the Company’s Series A‑1 preferred stock at an exercise price of $1.00 per share. The fair value of the warrant at the issuance date was recorded as a reduction to face value of the debt balance and was amortized as interest expense, along with other debt issuance costs, over the term of the loan. Due to the liquidation preferences of the Series A‑1 preferred stock, the Company recorded the warrant as a liability on the consolidated balance sheets. Following a reverse stock split in January 2016 and the closing of the IPO in February 2016, such warrant converted into a warrant to purchase 23,076 shares of common stock, which Silicon Valley Bank fully exercised in February 2016 pursuant to a net exercise provision for an aggregate of 19,271 shares of common stock.
Stock-based Compensation Expense	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. B ecause there had been no public market for the Company’s common stock prior to the IPO, there is 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 . Audit and Accounting Practice Aid Series: Valuation of Privately Held Company Equity Securities Issued as Compensation
Income taxes	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 Other income (expense), net consists 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. Prior to 2016, other income (expense), net included re‑measurement gains or losses associated with changes in the fair value of the tranche rights associated with the Series A‑1 preferred stock and anti‑dilutive protection liability associated with the issuance of common stock to certain licensors. In June 2015, upon the issuance of the final tranche of Series A preferred stock, the tranche right liability was settled and reclassified to Series A preferred stock and the anti‑dilutive protection liability was settled and reclassified to additional paid‑in‑capital. Therefore no further re‑measurement gains or losses will be recognized related to the tranche rights or the anti‑dilutive protection liability.
Comprehensive Loss	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. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements.
Concentrations of Credit Risk and Off-Balance Sheet Risk	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 and cash equivalents and accounts receivable. The Company’s cash and cash equivalents 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 (Note 8) for which the Company does not obtain collateral. As of December 31, 2016, all of the Company’s revenue to date had been generated exclusively from the collaboration with Juno Therapeutics and its award agreement with CFFT.
Segment Information	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.
Subsequent Events	Subsequent Events The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure.
Recent Accounting Pronouncements	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, 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 Company 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. Early adoption is permitted beginning after December 15, 2016, including interim reporting periods within those years. 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 two revenue arrangements, its license and collaboration with Juno Therapeutics and its arrangement with CFFT pursuant to which it has recognized a total of $5.7 million and $0.3 million, respectively, through December 31, 2016. 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 two arrangements. This analysis includes, but is not limited to, reviewing variable consideration as it relates to its agreements, reviewing the method and timing of recognition of the license payment, research funding and the $2.5 million milestone received from Juno Therapeutics, assessing potential disclosures and evaluating the impact of each potential method of adoption on the Company’s consolidated financial statements. In addition, the Company continues to monitor additional changes, modifications, clarifications or interpretations undertaken by the FASB, which may impact its conclusions. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements - Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (“ASU 2014-15”). ASU 2014-15 is intended to define management’s responsibility to evaluate whether there is substantial doubt about an organization’s ability to continue as a going concern and to provide related footnote disclosures, if required. ASU 2014-15 is effective for annual reporting periods ending after December 15, 2016, and applies to annual and interim periods thereafter. The Company adopted this standard during the three months ended December 31, 2016. The Company completed its evaluation and determined there was not substantial doubt about the organization’s ability to continue as a going concern. 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 impact that this ASU may have on its consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting , which simplifies share-based payment accounting through a variety of amendments. The standard will be effective for annual reporting periods and interim periods within those annual periods, beginning after December 15, 2016, and early adoption is permitted. The new standard became effective for the Company on January 1, 2017 and is not expected to have a material impact on the consolidated financial statements. In October 2016, the FASB issued ASU 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 is effective for fiscal years beginning after December 15, 2017, and interim periods within those years. Early adoption is permitted. The Company is evaluating the potential impact that the adoption of ASU 2016-18 will have on the Company’s financial position or results of operations.
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 and warrants 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. Contingently issuable shares of common stock pursuant to the 2016 License Agreements and Notes (Note 7) are excluded from the calculation of basic and diluted net loss per share calculation as the Payment Conditions have not been satisfied.

Summary of significant accoun25

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2016
Summary of significant accounting policies
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
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, 2016 2015 Redeemable convertible preferred stock — Warrant to purchase redeemable convertible preferred stock — Unvested restricted common stock Outstanding stock options Total

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Measurements
Schedule of assets and liabilities measured at fair value on a recurring basis	Assets 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 $ $ — $ — Money market funds, included in restricted cash and other non-current assets — — Total financial assets $ $ $ — $ — Assets and liabilities measured at fair value on a recurring basis as of December 31, 2015 are as follows (in thousands): Quoted Prices Significant in Active Other Significant Markets for Observable Unobservable December 31, Identical Assets Inputs Inputs Financial Assets 2015 (Level 1) (Level 2) (Level 3) Cash and cash equivalents $ $ $ — $ — Money market funds, included in other current assets — — Total financial assets $ $ $ — $ — Financial Liabilities Warrant liability $ $ — $ — $ Total financial liabilities $ $ — $ — $
Summary of the changes in the fair value of the Company’s Level 3 financial liabilities	The following table provides a roll‑forward of the fair value of the assets and liabilities measured at fair value on a recurring basis using Level 3 significant unobservable inputs (in thousands): Warrant Liability Balance at December 31, 2015 $ Changes in fair value, included in other income (expense), net Reclassification to additional paid-in capital in connection with the initial public offering Balance at December 31, 2016 $ —

Prepaid Expenses and Other Cu27

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 Prepaid expenses $ $ Restricted cash — Other Total $ $

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2016
Property and Equipment, Net
Schedule of property and equipment, net	Property and equipment, net consisted of the following (in thousands): As of December 31, 2016 2015 Building $ $ — Laboratory equipment Computer equipment Furniture and office equipment Leasehold improvements Software — Total property and equipment Less: accumulated depreciation Property and equipment, net $ $

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2016
Accrued Expenses
Schedule of accrued expenses	Accrued expenses consisted of the following (in thousands): As of December 31, 2016 2015 Patent and license fees $ $ Deferred initial public offering costs — Employee compensation costs Professional services Research and development — Other Total $ $

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
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 2017 $ 2018 2019 2020 2021 2022 and thereafter Total minimum lease payments $

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2016
Common Stock
Schedule of shares reserved for future issuance	As of December 31, 2016 2015 Shares reserved for redeemable convertible preferred stock outstanding — Shares reserved for future issuances of redeemable convertible preferred stock warrants — Shares reserved for outstanding stock option awards under the 2013 Stock Incentive Plan, as amended Remaining shares reserved, but unissued, for future awards under the 2013 Stock Incentive Plan, as amended — Shares reserved for outstanding stock option awards under the 2015 Stock Incentive Plan — Remaining shares reserved, but unissued, for future awards under the 2015 Stock Incentive Plan — Remaining shares reserved, but unissued, for future awards under the 2015 Employee Stock Purchase Plan —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
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 and comprehensive loss was as follows (in thousands): Year Ended December 31, 2016 2015 2014 Research and development $ $ $ General and administrative — Total stock-compensation expense $ $ $
Schedule of changes in unvested restricted stock	Weighted Average Grant Date Fair Value Shares Per Share Unvested Restricted Common Stock as of December 31, 2015 $ Issued — — Vested $ Unvested Restricted Common Stock as of December 31, 2016 $
Schedule of stock option activity	Weighted Average Remaining Aggregate Intrinsic Shares Exercise Price Contractual Life Value (in thousands) Outstanding at December 31, 2015 $ $ Granted $ — — Exercised $ — — Cancelled $ — — Outstanding at December 31, 2016 $ $ Vested and expected to vest at December 31, 2016 $ $ Exercisable at December 31, 2016 $ $
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, 2016 2015 2014 Expected volatility % % % Expected term (in years) Risk free interest rate % % % Expected dividend yield — — — There were 100,000 options granted to persons other than employees and directors during the year ended December 31, 2016. For the year ended December 31, 2016, 2015 and 2014, 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, 2016 2015 2014 Expected volatility % % % Expected term (in years) Risk free interest rate % % % Expected dividend yield — — —

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 Income tax computed at federal statutory tax rate % % State taxes, net of federal benefit % % General business credit carryovers % % Non-deductible expenses % % Change in valuation allowance % % — % — %
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, 2016 and 2015 (in thousands): Year Ended December 31, 2016 2015 Deferred tax assets: Net operating loss carryforwards $ $ Tax credit carryforwards Accrued expenses Capitalized patent costs Deferred revenue — Construction financing lease obligation — Other Total deferred tax assets Less valuation allowance Net deferred tax assets Deferred tax liabilities—depreciation and amortization Net deferred taxes $ — $ —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 Redeemable convertible preferred stock — Warrant to purchase redeemable convertible preferred stock — Unvested restricted common stock Outstanding stock options Total

Selected Quarterly Financial 35

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2016
Selected Quarterly Financial Data
Schedule of selected quarterly financial information	Three months ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (in thousands, except per share data) Total revenue $ $ $ $ Total operating expenses Total other expense, net Net loss $ $ $ $ Net loss applicable to common stockholders $ $ $ $ Net loss per share applicable to common stockholders — basic and diluted $ $ $ $ Three months ended March 31, 2015 June 30, 2015 September 30, 2015 December 31, 2015 (in thousands, except per share data) Total revenue $ — $ $ $ Total operating expenses Total other expense, net Net loss $ $ $ $ Net loss applicable to common stockholders $ $ $ $ Net loss per share applicable to common stockholders — basic and diluted $ $ $ $

Nature of Business (Details)

Nature of Business (Details) $ / shares in Units, $ in Thousands	Jan. 15, 2016	Feb. 29, 2016USD ($)$ / sharesshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares
Liquidity
Reverse stock split	0.385
Common stock, shares outstanding			35,818,131	3,233,638
Accumulated deficit | $			$ (185,526)	$ (88,348)
Initial Public Offering
Liquidity
Shares sold		6,785,000
Price to the public | $ / shares		$ 16
Aggregate net proceeds | $		$ 97,500
Shares issues upon conversion of preferred stock		24,929,709
Overallotment Option
Liquidity
Shares sold		885,000

Summary Of Significant Accoun37

Summary Of Significant Accounting Policies Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Restricted Cash and Investments [Abstract]
Restricted cash	$ 1,600	$ 300
Accounts Receivable
Bad debt write-offs	0	0
Allowance for doubtful accounts	$ 0	$ 0

Summary Of Significant Accoun38

Summary Of Significant Accounting Policies Property And Equipment (Details)	12 Months Ended
	Dec. 31, 2016
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 Accoun39

Summary Of Significant Accounting Policies Warrants (Details) $ / shares in Units, $ in Thousands

1 Months Ended

3 Months Ended

12 Months Ended

May 31, 2016USD ($)

Feb. 29, 2016shares

Dec. 31, 2016USD ($)

Sep. 30, 2016USD ($)

Jun. 30, 2016USD ($)

Mar. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Sep. 30, 2015USD ($)

Jun. 30, 2015USD ($)

Dec. 31, 2016USD ($)item

Dec. 31, 2015USD ($)

May 31, 2014$ / sharesshares

Warrants to purchase convertible preferred stock

Number of shares available for purchase under warrant | shares

23,076

Shares issued from exercise of warrants | shares

19,271

Revenue Recognition [Abstract]

Number of revenue agreements | item

2

Contracts Revenue

$ 898

$ 962

$ 3,388

$ 805

$ 792

$ 670

$ 167

$ 6,053

$ 1,629

Series A-1 redeemable convertible

Warrants to purchase convertible preferred stock

Number of shares available for purchase under warrant | shares

60,000

Exercise price | $ / shares

$ 1

Juno Therapeutics

Revenue Recognition [Abstract]

Contracts Revenue

5,700

$ 1,600

Juno Therapeutics | Collaboration and License Agreement

Revenue Recognition [Abstract]

Contracts Revenue

5,700

License Agreement Milestone Payment Received

$ 2,500

2,500

Cystic Fibrosis Foundation Therapeutics, Inc. | Development award agreement

Revenue Recognition [Abstract]

Contracts Revenue

$ 300

Fair Value Measurements Recurri

Fair Value Measurements Recurring Basis (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)item	Dec. 31, 2015USD ($)item
Financial Assets
Restricted cash		$ 320
Financial Liabilities
Transfers between levels | item	0	0
Recurring
Financial Assets
Cash and cash equivalents	$ 185,323	$ 143,180
Total financial assets	186,942	143,500
Financial Liabilities
Total financial liabilities		289
Recurring | Anti-dilution protection
Financial Liabilities
Warrant Liability		289
Recurring | Money market funds | Other current assets
Financial Assets
Restricted cash	1,619
Recurring | Money market funds | Other non-current assets
Financial Assets
Restricted cash		320
Recurring | Level 1
Financial Assets
Cash and cash equivalents	185,323	143,180
Total financial assets	186,942	143,500
Recurring | Level 1 | Money market funds | Other current assets
Financial Assets
Restricted cash	$ 1,619
Recurring | Level 1 | Money market funds | Other non-current assets
Financial Assets
Restricted cash		320
Recurring | Level 3
Financial Liabilities
Total financial liabilities		289
Recurring | Level 3 | Anti-dilution protection
Financial Liabilities
Warrant Liability		$ 289

Fair Value Measurements Assumpt

Fair Value Measurements Assumptions And Level 3 Rollforward (Details) - Warrant to purchase convertible redeemable preferred stock $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)
Roll-forward of the fair value of liabilities measured at fair value on a recurring basis
Balance, beginning of period	$ 289
Changes in fair value, included in other income (expense), net	87
Reclassification to additional paid-in capital in connection with IPO	$ (376)

Prepaid Expenses And Other Cu42

Prepaid Expenses And Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Prepaid Expenses and Other Current Assets
Prepaid expenses	$ 1,662	$ 460
Restricted cash		320
Other	110	6
Total	$ 1,772	$ 786

Property And Equipment Net (Det

Property And Equipment Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property and equipment disclosures
Property and equipment, gross	$ 41,934	$ 2,759
Less: accumulated depreciation	(1,556)	(629)
Property and equipment, net	40,378	2,130
Depreciation expense	1,202	471	$ 157
Building
Property and equipment disclosures
Property and equipment, gross	35,941
Laboratory equipment
Property and equipment disclosures
Property and equipment, gross	5,130	2,215
Computer equipment
Property and equipment disclosures
Property and equipment, gross	392	447
Furniture and office equipment
Property and equipment disclosures
Property and equipment, gross	170	74
Leasehold improvements
Property and equipment disclosures
Property and equipment, gross	200	$ 23
Software
Property and equipment disclosures
Property and equipment, gross	$ 101

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Accrued Expenses
Patent and license fees	$ 13,251	$ 3,395
Deferred initial public offering costs		283
Employee compensation costs	2,480	1,016
Professional services	729	382
Research and development	443
Other	536	380
Total	$ 17,439	$ 5,456

Commitments And Contingencies O

Commitments And Contingencies Operating Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Feb. 29, 2016USD ($)ft²	Nov. 30, 2015USD ($)
Operating Leased Assets [Line Items]
Collateral held		$ 320
Contractual obligation related to lease payments	$ 29,057
Rent expense	2,500	1,000	$ 900
Construction financing lease obligation, net of current portion	35,096
Facility Sublease Arrangement
Operating Leased Assets [Line Items]
Collateral held	$ 300	$ 300
Laboratory Sublease Cambridge
Operating Leased Assets [Line Items]
Contractual obligation related to lease payments					$ 1,900
Hurley Street, Cambridge, MA
Operating Leased Assets [Line Items]
Leased space ( in square feet) | ft²				59,783
Security deposit				$ 1,600
Estimated useful life	30 years
Rent expense	$ 500

Commitments And Contingencies A

Commitments And Contingencies Annual Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2016USD ($)
Future annual minimum lease payments
2,017	$ 3,957
2,018	4,055
2,019	4,155
2,020	4,257
2,021	4,362
2022 and after	8,271
Total minimum lease payments	$ 29,057

Commitments And Contingencies L

Commitments And Contingencies Licensor Expense Reimbursement (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Licensor Expense Reimbursement
Other Commitments [Line Items]
Payments for licensor expense reimbursement	$ 23.1	$ 9.4	$ 1.7

Commitments And Contingencies N

Commitments And Contingencies Notes Payable (Details) $ in Thousands	Dec. 31, 2016USD ($)
Notes Payable
Notes payable	$ 10,000
Broad and Wageningen University
Notes Payable
Notes payable	$ 10,000
Interest rate (as a percentage)	4.80%

Significant Agreements Juno The

Significant Agreements Juno Therapeutics Collaboration Agreement (Details) $ in Thousands	1 Months Ended			3 Months Ended							12 Months Ended
	Aug. 31, 2016USD ($)item	May 31, 2016USD ($)	May 31, 2015USD ($)item	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Collaboration and other research and development revenues				$ 898	$ 962	$ 3,388	$ 805	$ 792	$ 670	$ 167	$ 6,053	$ 1,629
Advance payment on future research and development activities				1,662				460			1,662	460
Research and development											56,979	18,846	$ 5,073
Deferred revenue, long-term				26,000				25,321			26,000	25,321
Accounts receivable				88				1,019			88	1,019
Juno Therapeutics
Collaboration and other research and development revenues											5,700	1,600
Deferred revenue				26,000				$ 25,300			26,000	$ 25,300
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
Number of additional licenses | item			3
Potential fee receivable for each gene target licensed			$ 2,500
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
Number of options to purchase Development and Commercialization License | item			3
Next possible milestone payment				$ 2,500							2,500
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											5,700
Collaboration and License Agreement | Adverum Biotechnologies, Inc
Additional fees	$ 1,000
Maximum number of inherited retinal diseases, to be treated | item	5
Up-front payment	$ 1,000
Adverum agreement consideration- pre-clinical activities	500
Advance payment on future research and development activities	$ 500
Research and development											500
Development award agreement | Cystic Fibrosis Foundation Therapeutics, Inc.
Agreement term		3 years
Potential research and development funding		$ 5,000
Collaboration and other research and development revenues											$ 300
Net sales threshold for potential milestone payments to be made		$ 100,000
Percentage of net sales threshold for potential milestone payments to be made		10.00%
Contract termination notice		90 days

Significant Agreements Other Ag

Significant Agreements Other Agreements (Details) $ in Thousands	Dec. 16, 2016USD ($)	Dec. 31, 2016USD ($)	Aug. 31, 2016USD ($)item	Oct. 31, 2014USD ($)shares	Aug. 31, 2014USD ($)shares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Notes payable		$ 10,000				$ 10,000
Research and development						56,979	$ 18,846	$ 5,073
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
Potential liability for future clinical and regulatory milestone payments related to product approval			4,900
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
Percentage of net sales threshold for potential milestone payments to be made			1.00%
Number of licensed products | item			4
Success Payments			$ 6,000
Potential liability for future sales milestone payments					$ 1,800
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
Notes payable		$ 10,000				10,000
License Agreement Contract Termination Term		4 months
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Cpf1 Market Cap Success Payments
Notes payable payment terms		150 days
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Maximum | Cpf1 License Agreement
Upfront fee		$ 16,500
Royalities 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		$ 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
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 clinical and regulatory milestone payments related to product approval	3,700
Potential liability for future sales milestone payments	13,500
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 clinical and regulatory milestone payments related to product approval	1,100
Potential liability for future sales milestone payments	$ 9,000
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
Duke University | License Intellectual Property and Technology
Upfront fee				100
Potential liability for future clinical, regulatory and commercial milestone payments related to product approval				$ 600

Preferred Stock (Details)

Preferred Stock (Details)	1 Months Ended								12 Months Ended
	Feb. 29, 2016shares	Aug. 31, 2015USD ($)shares	Jun. 30, 2015USD ($)shares	Nov. 30, 2014USD ($)shares	Oct. 31, 2014USD ($)shares	Jul. 31, 2014USD ($)shares	May 31, 2014USD ($)shares	Nov. 30, 2013USD ($)itemshares	Dec. 31, 2015USD ($)shares	Dec. 31, 2014USD ($)shares	Dec. 31, 2016shares	Feb. 08, 2016shares
Shares issued upon conversion (in shares)	24,929,709
Authorized common stock									92,000,000		195,000,000	195,000,000
Authorized preferred stock									0		5,000,000	5,000,000
Preferred stock, shares issued									0		0
Preferred stock, shares outstanding									0		0
Series A
Other Expenses | $									$ 35,500,000	$ 1,000,000
Series A | Preferred Stock Agreement
Number of Tranches | item								3
Series A | Preferred Stock Agreement | Maximum
Purchase price of convertible preferred stock | $								$ 43,000,000
Series A-1
Purchase price of convertible preferred stock | $				$ 13,000,000	$ 500,000	$ 2,500,000	$ 2,000,000	$ 3,300,000		$ 17,980,000
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and issuance costs (in shares)				13,000,000	500,000	2,500,000	2,000,000	3,260,000		18,000,000
Series A-2
Purchase price of convertible preferred stock | $			$ 21,700,000						$ 21,989,000
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and issuance costs (in shares)			16,698,672						16,890,699
Series A-2 | Executive
Purchase price of convertible preferred stock | $			$ 300,000
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and issuance costs (in shares)			192,027
Series B redeemable convertible
Purchase price of convertible preferred stock | $		$ 120,000,000							$ 119,722,000
Issuance of redeemable convertible preferred stock, net of preferred stock tranche liability and issuance costs (in shares)		26,666,660							26,666,660

Common Stock (Details)

Common Stock (Details) $ in Thousands	12 Months Ended	36 Months Ended
	Dec. 31, 2016Voteshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015shares
Common Stock, Capital Shares Reserved for Future Issuance	6,309,915	6,309,915	29,475,462
Redeemable Convertible Preferred Stock.
Common Stock, Capital Shares Reserved for Future Issuance			24,929,709
Common Stock.
Voting rights per share | Vote	1
Dividends, Common Stock | $		$ 0
Warrant to purchase convertible redeemable preferred stock
Common Stock, Capital Shares Reserved for Future Issuance			23,076
2013 Plan
Common Stock, Capital Shares Reserved for Future Issuance	1,595,082	1,595,082
2013 Plan | Common Stock.
Common Stock, Capital Shares Reserved for Future Issuance			2,848,630
2013 Plan | Common Stock. | Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance	1,595,082	1,595,082	1,674,047
2015 Plan
Common Stock, Capital Shares Reserved for Future Issuance	2,760,472	2,760,472
2015 Plan | Common Stock.
Common Stock, Capital Shares Reserved for Future Issuance	2,760,472	2,760,472
2015 Plan | Common Stock. | Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance	1,569,746	1,569,746
2015 Employee Stock Purchase Plan
Common Stock, Capital Shares Reserved for Future Issuance	384,615	384,615

Stock Based Compensation 2013 S

Stock Based Compensation 2013 Stock Incentive Plan (Details)	1 Months Ended				3 Months Ended	12 Months Ended	40 Months Ended
	Jul. 31, 2015shares	Apr. 30, 2015shares	Jun. 30, 2014shares	Sep. 30, 2013shares	Dec. 31, 2013shares	Dec. 31, 2016shares	Dec. 31, 2016itemshares	Dec. 31, 2015shares
Stock-based compensation disclosures
Shares reserved for future awards						6,309,915	6,309,915	29,475,462
Employee and Consultant Options
Stock-based compensation disclosures
Options granted (in shares)				0	0	1,794,746
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
Shares reserved for future awards						1,595,082	1,595,082
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
Stock-based compensation disclosures
Shares available for grant						1,569,746	1,569,746
Shares reserved for future awards						2,760,472	2,760,472
2015 Employee Stock Purchase Plan
Stock-based compensation disclosures
Shares issued						0
Shares reserved for future awards						384,615	384,615
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 Founde

Stock Based Compensation Founder Awards (Details) - Founder Awards $ / shares in Units, $ in Thousands	1 Months Ended
	Jun. 30, 2014USD ($)itemshares	Sep. 30, 2013$ / sharesshares
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%
Unvested share repurchase price (per share) | $ / shares		$ 0.0003
Number of founders not providing service | item	1
Shares repurchased	285,457
Payments for repurchase of shares | $	$ 74

Stock Based Compensation Expens

Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock-based compensation disclosures
Compensation expense	$ 16,881	$ 3,513	$ 55
Research and development.
Stock-based compensation disclosures
Compensation expense	4,234	3,015	$ 55
General and administrative
Stock-based compensation disclosures
Compensation expense	$ 12,647	$ 498

Stock Based Compensation - Rest

Stock Based Compensation - Restricted Stock and Stock Options (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in unvested stock options
Compensation expense			$ 16,881,000	$ 3,513,000	$ 55,000
Outstanding stock options
Changes in unvested stock options
Fair value of employee options vested			$ 2,800,000	$ 8,000	$ 0
Restricted Stock
Changes in unvested restricted stock
Unvested restricted common stock, beginning of period (in shares)			1,596,853
Vested (in shares)			(774,215)
Unvested restricted common stock, end of period(in shares)			822,638	1,596,853
Weighted Average Grant Date Fair Value
Balance, beginning of period			$ 0.0188
Vested (in dollars per share)			0.0162
Balance, beginning of period			$ 0.0213	$ 0.0188
Restricted Stock | Employees
Changes in unvested stock options
Compensation expense			$ 0	$ 0
Unrecognized stock-based compensation expense			0
Restricted Stock | Non-employees
Changes in unvested stock options
Compensation expense			8,300,000	$ 2,300,000
Unrecognized stock-based compensation expense			$ 3,400,000
Period for recognition			7 months 6 days
Employee and Consultant Options
Changes in unvested stock options
Outstanding, beginning of period (in shares)			1,713,385
Granted (in shares)	0	0	1,794,746
Exercised (in shares)			(76,298)		75,304
Cancelled (in shares)			(20,050)
Outstanding, end of period (in shares)			3,411,783	1,713,385
Vested and expected to vest (in shares)			3,356,306
Exercisable (in shares)			568,617
Outstanding, beginning of period (in dollars per share)			$ 6.31
Granted (in dollars per share)			20.24
Exercised (in dollars per share)			3.04		$ 0.03
Cancelled (in dollars per share)			6.74
Outstanding, end of period (in dollars per share)			13.71	$ 6.31
Vested and expected to vest outstanding, weighted average exercise price (in dollars per share)			13.71
Exercisable (in dollar per share)			$ 6.86
Remaining contractual life			8 years 9 months 18 days	9 years 7 months 6 days
Vested and expected to vest, remaining contractual life			8 years 9 months 18 days
Exercisable, remaining contractual life			8 years 7 months 6 days
Aggregate intrinsic value			$ 16,190,000	$ 15,580,000
Vesting and expected to vest, aggregate intrinsic value			15,926,000
Exercisable, aggregated intrinsic value			$ 5,330,000
Unvested stock options (in shares)			21,955	39,338
Intrinsic value of options exercised			$ 900,000	$ 100,000	$ 0
Weighted average fair value of options granted (per share)			$ 14.10	$ 5.91	$ 0.03
Employee and Consultant Options | Employees
Changes in unvested stock options
Compensation expense			$ 6,000,000	$ 700,000	$ 0
Unrecognized stock-based compensation expense			$ 24,500,000
Period for recognition			3 years
Employee and Consultant Options | Non-employees
Changes in unvested stock options
Granted (in shares)			100,000

Stock Based Compensation Assump

Stock Based Compensation Assumptions (Details) - Employee and Consultant Options - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employees and directors
Assumptions
Expected volatility	78.40%	78.80%	87.60%
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Risk free interest rate	1.50%	1.70%	1.90%
Employees
Assumptions
Unrecognized stock-based compensation expense	$ 24.5
Period for recognition	3 years
Non-employees
Assumptions
Expected volatility	76.50%	80.00%	80.50%
Expected term (in years)	10 years	10 years	9 years 6 months
Risk free interest rate	1.60%	2.20%	1.50%

401(K) Savings Plan (Details)

401(K) Savings Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Contributions to the 401(k) Plan		$ 0
Subsequent Event
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%

Income Taxes - Effective Income

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

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets And Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 16,490	$ 11,466
Tax credit carryforwards	2,014	730
Accrued expenses	7,353	1,652
Capitalized patent costs	16,025	5,985
Deferred revenue	9,672
Construction financing lease obligation	13,685
Other	2,979	242
Total deferred tax assets	68,218	20,075
Less valuation allowance	(54,300)	(19,938)
Net deferred tax assets	13,918	137
Deferred tax liabilities - depreciation and amortization	$ (13,918)	$ (137)

Income Taxes - Net Operating Lo

Income Taxes - Net Operating Losses (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Net operating losses	$ 82.4	$ 58.3
NOL carryforward from stock-based compensation	$ 0.7	$ 0.1

Income Taxes - Tax Credit Carry

Income Taxes - Tax Credit Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Research and development
Tax Credit Carryforward [Line Items]
Tax credit carryforward	$ 2.3	$ 0.8

Income Taxes - Other Narrative

Income Taxes - Other Narrative Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Section 382 period	3 years
Section 382 percentage	50.00%
Valuation allowance	$ 54,300	$ 19,938
Change in the valuation allowance	34,400
Unrecognized tax benefits	$ 0	$ 0

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares	1 Months Ended	12 Months Ended
	Feb. 29, 2016	Dec. 31, 2016	Dec. 31, 2015
Potentially dilutive securities
Anti-dilutive common stock equivalent shares		4,234,421	28,263,023
Redeemable Convertible Preferred Stock.
Potentially dilutive securities
Anti-dilutive common stock equivalent shares			24,929,709
Warrant to purchase convertible redeemable preferred stock
Potentially dilutive securities
Anti-dilutive common stock equivalent shares			23,076
Restricted Stock
Potentially dilutive securities
Anti-dilutive common stock equivalent shares		822,638	1,596,853
Outstanding stock options
Potentially dilutive securities
Anti-dilutive common stock equivalent shares		3,411,783	1,713,385
Initial Public Offering
Potentially dilutive securities
Shares sold	6,785,000
Shares issues upon conversion of preferred stock	24,929,709

Related-Party Transactions (Det

Related-Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Rent and facility related fees
Related Party Transaction [Line Items]
Payments to related party	$ 1.4	$ 1.2
Professional fees
Related Party Transaction [Line Items]
Payments to related party		$ 0.1	$ 0.2

Selected Quarterly Financial 66

Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Data
Total revenue	$ 898	$ 962	$ 3,388	$ 805	$ 792	$ 670	$ 167		$ 6,053	$ 1,629
Total operating expenses	39,882	22,127	22,588	18,644	13,165	8,052	10,563	$ 5,161	103,241	36,941	$ 12,723
Total other expense, net	(392)	145	158	94	(260)	(23)	(37,175)	(130)	5	(37,588)	(962)
Net loss and comprehensive loss	(39,376)	(21,020)	(19,042)	(17,745)	(12,633)	(7,405)	(47,571)	(5,291)	(97,183)	(72,900)	(13,685)
Net loss applicable to common stockholders	$ (39,376)	$ (21,020)	$ (19,042)	$ (17,792)	$ (12,732)	$ (7,509)	$ (47,667)	$ (5,386)	$ (97,230)	$ (73,294)	$ (13,994)
Net loss per share applicable to common stockholders - basic and diluted	$ (1.10)	$ (0.59)	$ (0.54)	$ (0.80)	$ (4.05)	$ (2.57)	$ (21.45)	$ (2.75)