CRISPR Therapeutics (CRSP) 10-K 8 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 28, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	CRSP
Entity Registrant Name	CRISPR THERAPEUTICS AG
Entity Central Index Key	1,674,416
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		46,976,000
Entity Public Float			$ 605.5

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash	$ 239,758	$ 315,520
Accounts receivable, including related party amounts of $821 and $752 as of December 31, 2017 and 2016, respectively	2,626	3,157
Prepaid expenses and other current assets, including related party amounts of $1,871 and $0 as of December 31, 2017 and 2016, respectively	6,001	1,511
Total current assets	248,385	320,188
Property and equipment, net	18,857	21,027
Intangible assets, net	344	399
Restricted cash	3,154	3,150
Other non-current assets	606	198
Total assets	271,346	344,962
Current liabilities:
Accounts payable	1,639	4,569
Accrued expenses, including related party amounts of $0 and $537 as of December 31, 2017 and 2016, respectively	11,361	16,320
Accrued tax liabilities	347	23
Deferred rent	1,027	1,027
Other current liabilities	137	59
Total current liabilities	14,511	21,998
Deferred revenue, including related party amounts of $91 and $527 as of December 31, 2017 and 2016, respectively	56,928	77,646
Deferred rent non-current	11,761	12,283
Other non-current liabilities	314	189
Total liabilities	83,514	112,116
Commitments and contingencies (Note 8)
Shareholders’ equity (deficit):
Common shares, CHF 0.03 par value, 41,092,969 and 40,253,674 shares authorized at December 31, 2017 and 2016, respectively, 41,037,121 and 40,164,307 shares issued at December 31, 2017 and 2016, respectively, 40,592,248, and 39,719,434 shares outstanding at December 31, 2017 and 2016, respectively, 16,419,632 and 15,325,607 shares in conditional capital at December 31, 2017 and 2016, respectively.	1,240	1,216
Treasury shares, at cost, 444,873 shares and no shares at December 31, 2017 and 2016, respectively	0	0
Additional paid-in capital	312,018	288,739
Accumulated deficit	(125,440)	(57,083)
Accumulated other comprehensive income (loss)	14	(26)
Total shareholders’ equity	187,832	232,846
Total liabilities and shareholders’ equity	$ 271,346	$ 344,962

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) $ in Thousands	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares
Statement Of Financial Position [Abstract]
Accounts receivable, related party amount, current | $	$ 821	$ 752
Prepaid expenses and other current assets, related party amount | $	1,871	0
Accrued expenses related party amount, current | $	0	537
Deferred revenue related party amount | $	$ 91	$ 527
Common stock, shares authorized	41,092,969	40,253,674
Common stock, shares issued	41,037,121	40,164,307
Common stock, shares outstanding	40,592,248	39,719,434
Common stock, conditional capital	16,419,632	15,325,607
Treasury stock, shares	444,873	0

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	[1]	$ 40,997	$ 5,164	$ 247
Operating expenses:
Research and development	[2]	69,800	42,238	12,573
General and administrative		35,845	31,056	13,403
Total operating expenses		105,645	73,294	25,976
Loss from operations		(64,648)	(68,130)	(25,729)
Other (expense) income:
Interest expense			(8,050)	(108)
Loss from equity method investment		(1,763)	(36,532)
Gain on extinguishment of convertible loan			11,482
Other (expense) income, net		(197)	78,512	16
Total other (expense) income, net		(1,960)	45,412	(92)
Net loss before provision for from income taxes		(66,608)	(22,718)	(25,821)
Provision for income taxes		(1,749)	(484)	(7)
Net loss		(68,357)	(23,202)	(25,828)
Foreign currency translation adjustment		40	(18)	(6)
Comprehensive loss		(68,317)	(23,220)	(25,834)
Reconciliation of net loss to net loss attributable to common shareholders:
Net loss		(68,357)	(23,202)	(25,828)
Loss attributable to noncontrolling interest			25	325
Net loss attributable to common shareholders		$ (68,357)	$ (23,177)	$ (25,503)
Net loss per share attributable to common shareholders—basic and diluted		$ (1.71)	$ (1.89)	$ (5.06)
Weighted-average common shares outstanding used in net loss per share attributable to common shareholders—basic and diluted		40,057,365	12,257,483	5,037,404
[1]	(1)  Including the following amounts of revenue from a related party, see Note 16:
[2]	(2)  Including the following amounts of research and development from a related party, see Note 16:

Consolidated Statements of Ope5

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue from related party	$ 4,760	$ 1,190
Research and development from related party	$ 4,523	$ 1,755	$ 1,055

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Shares and Shareholder's (Deficit) Equity - USD ($) $ in Thousands	Total	Series A-1 Redeemable Convertible Preferred Shares [Member]	Series A-2 Redeemable Convertible Preferred Shares [Member]	Series A-3 Redeemable Convertible Preferred Shares [Member]	Series B Redeemable Convertible Preferred Shares [Member]	Common Shares [Member]	Common Shares [Member]TRACR Hematology Limited [Member]	Treasury Shares [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]TRACR Hematology Limited [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total CRISPR Therapeutics AG Shareholders' (Deficit) Equity [Member]	Total CRISPR Therapeutics AG Shareholders' (Deficit) Equity [Member]TRACR Hematology Limited [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member]TRACR Hematology Limited [Member]
Beginning balance at Dec. 31, 2014		$ 1,169	$ 5,101
Beginning balance (in shares) at Dec. 31, 2014		440,001	3,120,001
Beginning balance at Dec. 31, 2014	$ (6,974)					$ 120			$ 1,168		$ (8,403)	$ (2)	$ (7,117)		$ 143
Beginning balance (in shares) at Dec. 31, 2014						3,559,985
Receipt of preferred shares subscription receivable			$ 5,293
Issuance of preferred shares net of issuance cost and subscription receivable				$ 22,518
Issuance of preferred shares net of issuance cost and subscription receivable (in shares)				10,758,006
Adjustment to noncontrolling interest upon share exchange transaction							$ 61			$ 1				$ 62		$ (62)
Adjustment to noncontrolling interest upon share exchange transaction (in shares)							1,968,094
Issuance of shares, net of issuance cost					$ 30,440
Issuance of shares, net of issuance cost (in shares)					4,519,016
Equity-based compensation expense	3,684								3,467				3,467		217
Other comprehensive income (loss)	(6)											(6)	(6)
Net loss	$ (25,828)										(25,503)		(25,503)		(325)
Ending balance at Dec. 31, 2015		$ 1,169	$ 10,394	$ 22,518	$ 30,440
Ending balance (in shares) at Dec. 31, 2015	18,837,024	440,001	3,120,001	10,758,006	4,519,016
Ending balance at Dec. 31, 2015	$ (29,124)					$ 181			4,636		(33,906)	(8)	(29,097)		(27)
Ending balance (in shares) at Dec. 31, 2015						5,528,079
Conversion of Convertible Loans					$ 61,929
Conversion of Convertible Loans (in shares)					5,464,608
Receipt of preferred shares net of issuance cost and subscription receivable				$ 22,850
Issuance of preferred shares net of issuance cost and subscription receivable					$ 36,265
Issuance of preferred shares net of issuance cost and subscription receivable (in shares)					2,834,252
Conversion of redeemable convertible preferred shares into common share, value	185,565	$ (1,169)	$ (10,394)	$ (45,368)	$ (128,634)	$ 823			184,742				185,565
Conversion of redeemable convertible preferred shares into common share (in shares)		(440,001)	(3,120,001)	(10,758,006)	(12,817,876)	27,135,884
Adjustment to noncontrolling interest upon share exchange transaction							$ 10			$ (62)				$ (52)		$ 52
Adjustment to noncontrolling interest upon share exchange transaction (in shares)							328,017
Issuance of shares, net of issuance cost	88,664					$ 213			88,451				88,664
Issuance of shares, net of issuance cost (in shares)						7,100,000
Repurchase of treasury shares, value						$ (13)			13
Repurchase of treasury shares (in shares)						(444,873)		444,873
Vesting of restricted shares, value	82					$ 1			81				82
Vesting of restricted shares (in shares)						53,427
Exercised of vested options, value	35					$ 1			34				35
Exercised of vested options(in shares)						18,900
Equity-based compensation expense	10,844								10,844				10,844
Other comprehensive income (loss)	(18)											(18)	(18)
Net loss	(23,202)										(23,177)		(23,177)		$ (25)
Ending balance (in shares) at Dec. 31, 2016		0	0	0	0
Ending balance at Dec. 31, 2016	232,846					$ 1,216			288,739		(57,083)	(26)	232,846
Ending balance (in shares) at Dec. 31, 2016						39,719,434		444,873
Vesting of restricted shares, value	59					$ 1			58				59
Vesting of restricted shares (in shares)						33,519
Exercised of vested options, value	2,608					$ 23			2,585				2,608
Exercised of vested options(in shares)						839,295
Equity-based compensation expense	20,636								20,636				20,636
Other comprehensive income (loss)	40											40	40
Net loss	(68,357)										(68,357)		(68,357)
Ending balance (in shares) at Dec. 31, 2017		0	0	0	0
Ending balance at Dec. 31, 2017	$ 187,832					$ 1,240			$ 312,018		$ (125,440)	$ 14	$ 187,832
Ending balance (in shares) at Dec. 31, 2017						40,592,248		444,873

Consolidated Statements of Red7

Consolidated Statements of Redeemable Convertible Preferred Shares and Shareholder's (Deficit) Equity (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Common stock, issuance cost	$ 8,300
Series A-3 Redeemable Convertible Preferred Shares [Member]
Preferred shares, issuance cost		$ 332
Preferred shares, subscription receivable		22,850
Series B Redeemable Convertible Preferred Shares [Member]
Preferred shares, issuance cost	$ 1,800	$ 38

Consolidated Statements of Cash

Consolidated Statements of Cash Flows $ in Thousands, SFr in Millions	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Operating activities
Net loss	$ (68,357)	$ (23,202)	$ (25,828)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	3,024	925	127
Equity-based compensation	18,873	10,844	3,684
Non-cash interest		8,050	97
Unrealized foreign currency remeasurement (gain) loss	(9)	2	(20)
Gain on extinguishment of convertible loan		(11,482)
Other income - formation of joint venture		(78,608)
Loss from equity method investment	1,763	36,380
Changes in:
Restricted cash	(4)	(2,450)	(650)
Accounts receivable	531	(2,818)	(339)
Prepaid expenses and other assets	(4,117)	(1,071)	(620)
Accounts payable and accrued expenses	(831)	3,860	7,708
Deferred revenue	(20,718)	1,917	75,090
Deferred rent	(522)	2,360	165
Other liabilities, net	270	(17)	14
Net cash (used in) provided by operating activities	(70,097)	(55,310)	59,428
Investing activities
Purchase of property and equipment	(7,814)	(3,016)	(1,154)
Proceeds from contribution of intellectual property to equity method investee		35,000
Cash investment in equity method investee		(100)
Purchase of available for sale debt security	(500)
Net cash (used in) provided by investing activities	(8,314)	31,884	(1,154)
Financing activities
Proceeds from issuance of common shares in IPO, net of issuance costs		54,061
Proceeds from issuance of common shares in private placement		35,000
Proceeds from exercise of options	2,608	34
Proceeds from issuance of restricted shares			243
Issuance costs for preferred share financings		(1,810)	(370)
Proceeds from issuance of convertible loans		35,010	38,239
Net cash provided by financing activities	2,608	183,220	96,733
Effect of exchange rate changes on cash	41	(235)	9
(Decrease) increase in cash	(75,762)	159,559	155,016
Cash, beginning of period	315,520	155,961	945
Cash, end of period	239,758	315,520	155,961
Supplemental disclosure of non-cash investing and financing activities
Property and equipment purchases in accounts payable and accrued expenses		7,014	246
Costs for proposed supplemental offering in accounts payable and accrued expenses	$ 290
Property and equipment related to lease incentives		10,785
Conversion of preferred shares to common shares upon IPO		185,565
Conversion of Vertex and Bayer convertible loans and accrued interest		61,929
Issuance costs for public offering in accounts payable and accrued expenses		397
Contribution of intellectual property to Casebia		36,380
Series A-2 Redeemable Convertible Preferred Shares [Member]
Financing activities
Proceeds from issuance of preferred shares			5,293
Series A-3 Redeemable Convertible Preferred Shares [Member]
Financing activities
Proceeds from issuance of preferred shares		22,850	22,850
Series B Redeemable Convertible Preferred Shares [Member]
Financing activities
Proceeds from issuance of preferred shares		$ 38,075	$ 30,478

Organization and Operations

Organization and Operations	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Operations	1. Organization and Operations Nature of business CRISPR Therapeutics AG (“CRISPR” or the “Company”) was formed on October 28, 2013 in Basel, Switzerland. In 2017, the Company changed its registered office to Zug, Switzerland. The Company was established to translate CRISPR/Cas9, a genome editing technology, into transformative gene-based medicines for the treatment of serious human diseases. The foundational intellectual property underlying the Company’s operations was licensed to the Company and its subsidiaries in April 2014. The Company devotes substantially all of its efforts to product research and development activities, initial market development and raising capital. The Company’s principal offices are headquartered in Zug, Switzerland and operations are in Cambridge, Massachusetts. 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. The Company had an accumulated deficit of $125.4 million as of December 31, 2017 and has financed its operations to date from proceeds obtained from its initial public offering, a series of preferred shares and convertible loan issuances, and upfront fees received under its collaboration and joint venture arrangements. The Company will require substantial additional capital to fund its research and development and ongoing operating expenses. Liquidity In January 2018, the Company completed an offering of 5,750,000 shares of common shares, which were sold at a price of $22.75 per share. This offering resulted in $122.6 million of net proceeds to the Company. Refer to Note 17 “Subsequent Events” in the accompanying notes to the consolidated financial statements for further details regarding the January 2018 sale of common stock. The Company believes its cash of $239.8 million at December 31, 2017 and the proceeds from the January 2018 offering will be sufficient to fund the Company’s current operating plan for at least the next 24 months. Thereafter, the Company will be required to obtain additional funding. There can be no assurances, however, that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all.

Summary of Significant Accounti

Summary of Significant Accounting Policies and Basis of Presentation	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies and Basis of Presentation	2. Summary of Significant Accounting Policies and basis of presentation Basis of Presentation and Use of Estimates The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”), and include the accounts of (i) the Company, (ii) its wholly-owned subsidiaries, CRISPR Therapeutics Ltd., CRISPR Therapeutics Inc., and TRACR Hematology Inc., as of December 31, 2017. All intercompany accounts and transactions have been eliminated. 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 (“ASUs”) of the Financial Accounting Standards Board (“FASB”). Investments in partnerships where the Company has significant influence because it has a voting interest of 20% to 50%, are accounted for under the equity method. Results of associated companies are presented on a one-line basis. The Company accounts for its 50% investment share of Casebia Therapeutics LLP (“Casebia”) under the equity method of accounting. See Note 9 for further details. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, equity-based compensation expense, revenue recognition, equity method investments, and reported amounts of expenses during the reported period. Significant estimates in these consolidated financial statements have been made in connection with the calculation of revenues, research and development expenses, valuation of equity method of investment, equity-based compensation expense, fair value of Common Shares, fair value of intangible assets, and the provision for or benefit from income taxes. The Company bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions. 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, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company and the Company’s chief operating decision maker, namely, the chief executive officer, view the Company’s operations and manage its business in one operating segment, which is the business of discovering, developing and commercializing therapies derived from or incorporating genome-editing technology. Foreign Currency Translation and Transactions The Company’s reporting currency is the U.S. Dollar. The Company‘s consolidated entities have the U.S. dollar as their functional currency with the exception of CRISPR Ltd. which has the British Pound Sterling (“GBP”) as its functional currency. CRISPR Ltd. has assets and liabilities translated into U.S. dollars at exchange rates in effect at the end of the year. Revenue and expenses are translated using the average exchange rates for the period. Net unrealized gains and losses resulting from foreign currency translation are included in accumulated other comprehensive income (loss), which is a separate component of shareholders’ (deficit) equity. Net foreign currency exchange transaction gains and losses resulting from the remeasurement of transactions denominated in currencies other than functional currency are included in other (expense) income, net in the consolidated statements of operations and comprehensive loss. Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of 90 days or less from the purchase date to be cash equivalents. As of December 31, 2017 and 2016, the Company had $239.8 million and $315.5 million in cash equivalents, respectively. All cash was held in depository accounts and is reported at fair value. Accounts Receivable Accounts receivable of $2.6 million at December 31, 2017 consist of receivables from Vertex Pharmaceuticals, Incorporated (“Vertex”) and Casebia. As of December 31, 2016, the Company had accounts receivable of $3.2 million consisting of receivables from Vertex. Accounts receivables are recorded at invoiced amounts due under both the Vertex and Casebia collaboration agreements (see Note 9). Vertex and Casebia are creditworthy entities that maintain an ongoing relationship with the Company, as such the Company did not have an allowance for estimated losses recorded related to these receivables. Concentrations of Credit Risk and Off-balance Sheet Risk Financial instruments that potentially subject the Company to concentrations of credit risk are primarily cash. The Company’s cash is held in accounts with financial institutions that management believes are creditworthy. 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. The Company has no financial instruments with off-balance sheet risk of loss. Fair Value of Financial Instruments The Company’s financial instruments consist of a convertible debt instrument, accounts payable, accrued expenses and other non-current liabilities. The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurement and Disclosures The accounting standard describes a fair value hierarchy based on three levels of inputs, of which the first two are considered observable and the last unobservable, that may be used to measure fair value, which are the following: Level 1 — Quoted prices in active markets that are accessible at the market date for identical unrestricted assets or liabilities. Level 2 — Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs for which all significant inputs are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. To the extent that 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 amount of accounts receivable, accounts payable, and accrued expenses as reported on the consolidated balance sheets as of December 31, 2017 and 2016, approximate fair value, due to the short-term duration of these instruments. The fair value of the Company’s equity method investment in Casebia and convertible debt instruments were determined using level 3 inputs (See Note 9). Property and Equipment Property and equipment 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. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation is recorded using the straight-line method over the estimated useful lives of the respective assets, which are as follows: Asset Estimated useful life Computer equipment 3 years Furniture, fixtures, and other 5 years Laboratory equipment 5 years Leasehold improvements Shorter of useful life or remaining lease term 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 value 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 value of the assets exceed their fair value. The Company has not recognized any impairment losses in the years ended December 31, 2017, 2016, and 2015. Revenue Recognition The Company recognizes revenue for each unit of accounting when all of the following criteria are met: (i) persuasive evidence of an arrangement exists, (ii) delivery has occurred or services have been rendered, (iii) the seller’s price to the buyer is fixed or determinable and (iv) collectability is reasonably assured. The terms of the Company’s collaboration and license agreements contain multiple deliverables, which include licenses to CRISPR/Cas9-based therapeutic products directed to specific targets, referred to as co-exclusive or exclusive licenses, joint steering committee participation, as well as research and development activities to be performed by the Company on behalf of the collaboration partner related to the licensed targets. Payments that the Company may receive under these agreements include nonrefundable technology access fees, payments for research activities, payments based upon the achievement of specified milestones and royalties on any resulting net product sales. The Company records the elements of its collaboration agreements that represent joint operating activities in accordance with FASB ASC Topic 808, Collaborative Arrangements (“ASC 808”). Accordingly, the elements of the collaboration agreements that represent activities in which both parties are active participants and to which both parties are exposed to the significant risks and rewards that are dependent on the commercial success of the activities, are recorded as collaborative arrangements. The Company considers the guidance in FASB ASC Topic 605-45, Revenue Recognition—Principal Agent Considerations (“ASC 605-45”) in determining the appropriate treatment for the transactions between the Company and its collaborative partner and the transactions between the Company and third parties. Generally, the classification of transactions under the collaborative arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. To date, the Company’s only source of revenue has been the collaboration and license and joint development and commercialization agreement with Vertex as well as research and development services provided to Casebia under the joint venture with Bayer HealthCare LLC (“Bayer”) (see Note 9). The Company evaluates multiple-element arrangements based on the guidance in FASB ASC Topic 605-25, Revenue Recognition—Multiple-Element Arrangements The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the selling price of a unit of accounting within each arrangement following the hierarchy of evidence prescribed by ASC 605-25. Accordingly, 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. The Company typically uses BESP to estimate the selling price as it generally does not have VSOE or TPE of selling price for its units of accounting. 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 periodically validates the BESP used 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 following criteria are met for that particular unit of accounting: 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. 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 contractual or estimated performance period for the undelivered items, 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 over 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. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company expects to complete its performance obligations under an arrangement. Steering committee services that are not inconsequential or perfunctory and that are determined to be performance obligations are combined with other research services or performance obligations required under an arrangement, if any, in determining the level of effort required in an arrangement and the period over which the Company expects to complete its aggregate performance obligations. 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: (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 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. 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. The Company will recognize revenue in its entirety upon successful accomplishment of any substantive milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, with a cumulative catch-up being recognized for the elapsed portion of the 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 Expenses Research and development costs, which include employee compensation costs, facilities, lab supplies and materials, overhead, preclinical development, and other related costs, are charged to expense as incurred. Research and development costs also include the costs the Company incurs in its performance of services or provision of materials in connection with the funded research undertaken as a part of the Company’s collaborative agreement with Vertex and Casebia. See Note 9 for further details. Operating Leases The Company leases office and laboratory facilities under a non-cancelable operating lease agreements. The lease agreements contain free or escalating rent payment provisions. The Company recognizes rent expense under such leases on a straight-line basis over the term of the lease with the difference between the expense and the payments recorded as deferred rent on the consolidated balance sheets. Lease renewal periods are considered on a lease-by-lease basis in determining the lease term. Funding of leasehold improvements by the Company’s landlord are accounted for as a tenant improvement allowance and are amortized as a reduction of rent expense over the term of the lease. Leasehold improvements are amortized straight-line over the shorter of the useful life or the remaining lease term. Equity Based Compensation Expense The Company recognizes equity-based compensation expense for awards of equity instruments to employees and non-employee directors based on the grant date fair value of those awards in accordance with FASB ASC Topic 718, Stock Compensation The Company accounts for stock options issued to non-employees under FASB ASC Topic 505-50, Equity Based Payments to Non-Employees The Black-Scholes option pricing model requires the input of certain subjective assumptions, including (i) the expected share price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate and (iv) the expected dividend yield. Due to the lack of a public market for the trading of the Company’s Common Shares prior to its IPO and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The group of representative companies have characteristics similar to the Company, including stage of product development and focus on the life science industry. The Company uses the simplified method, which is the average of the final vesting tranche date and the contractual term, to calculate the expected term for options granted to employees as it 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 assumption. The risk-free interest rate is 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 has no current plans to pay any dividends on its Common Shares. The Company expenses the fair value of its equity-based compensation awards granted to employees on a straight-line basis over the associated service period, which is generally the period in which the related services are received. The Company measures equity-based compensation awards granted to non-employees at fair value as the awards vest and recognizes the resulting value as compensation expense at each financial reporting period. The Company records the expense for equity-based compensation awards subject to performance-based milestone vesting over the remaining service period using the accelerated method when management determines that achievement of the milestone is probable. Management evaluates when the achievement of a performance-based milestone is probable based on the expected satisfaction of the performance conditions as of the reporting date. The Company use a Monte Carlo simulation option-pricing model to determine the fair value of market-based awards. The model uses the same input assumptions as the Black-Scholes model, yet, it also incorporates the possibility that the market condition may not be satisfied. Compensation cost related to market-based awards are recognized using the accelerated method regardless of whether the market condition is satisfied, provided that the requisite service has been provided. Patent Costs Costs to secure and prosecute patent application and other legal costs related to the protection of the Company’s intellectual property are expensed as incurred and are classified as general and administrative expenses in the Company’s consolidated statements of operations. Income Taxes Income taxes are recorded in accordance with FASB ASC Topic 740, Income Taxes The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes 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. As of December 31, 2017 and 2016, the Company does not have any significant uncertain tax positions. The Company’s practice is to recognize interest and/or penalties related to income tax matters in income tax expense. See Note 14 for further details. Comprehensive Loss Comprehensive loss consists of net income or loss and changes in equity during the period from transactions and other events and circumstances generated from non-owner sources. The Company’s net loss equals comprehensive loss, net of any changes in the foreign currency translation adjustment, for all periods presented. In addition, comprehensive loss attributable to the noncontrolling interest equals net loss for all periods presented. Variable Interest Entities The Company reviews each legal entity formed by parties related to the Company to determine whether or not the Company has a variable interest in the entity and whether or not the entity would meet the definition of a VIE in accordance with FASB ASC Topic 810, Consolidation If the Company determines it is the primary beneficiary of a VIE that meets the definition of a business, the Company measures the assets, liabilities and noncontrolling interests of the newly consolidated entity at fair value in accordance with FASB ASC Topic 805, Business Combinations In February 2016, Casebia Therapeutics LLP, a limited liability partnership, was formed in the United Kingdom. In March 2016 upon consummation of the JV, Bayer and the Company each received a 50% equity interest in the entity in exchange for their contributions to the entity. The Company determined that Casebia was considered a VIE and concluded that it is not the primary beneficiary of the VIE. As such, the Company did not consolidate Casebia’s results into the consolidated financial statements. See Note 4 for further details. Intangible Assets The Company’s intangible assets consist of acquired intellectual property rights and relate to the Company’s interest in TRACR. Intangible assets are recorded at fair value at the date of the business combination and are stated in the consolidated balance sheets net of accumulated amortization and impairments, if applicable. The Company evaluates the remaining useful life of intangible assets subject to amortization on a periodic basis to determine whether events and circumstances would indicate impairment or warrant a revision to the remaining useful life. If the estimate of an intangible asset’s remaining useful life is changed, the Company amortizes the remaining carrying value of the intangible asset prospectively over the revised remaining useful life. Intangible assets related to the acquired intellectual property rights are amortized over their estimated useful lives using the straight-line method as the pattern of revenues cannot be reasonably estimated. Amortization related to the acquired intellectual property rights is recorded in general and administrative expense in the consolidated statements of operations and comprehensive loss. Net Loss Per Share Attributable to Common Shareholders Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common shareholders by the weighted-average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common shareholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options and warrants using the treasury stock method. The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to do so would be anti-dilutive (in common stock equivalent shares): As of December 31, 2017 2016 2015 Convertible preferred shares — — 18,837,024 Conversion of convertible loans — — 4,110,987 Dr. Emmanuelle Charpentier call option — — 328,017 Outstanding options 6,262,339 4,535,371 1,939,986 Unvested unissued restricted shares 157,515 89,367 142,794 Total 6,419,854 4,624,738 25,358,808 Subsequent Events The Company considered the events or transactions occurring after the balance sheet date, but prior to the issuance of the consolidated financial statements, for potential recognition or disclosure in its consolidated financial statements. All significant subsequent events have been properly disclosed in the consolidated financial statements. Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”). Subsequently, the FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606), which adjusted the effective date of ASU 2014-09; ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which amends the principal-versus-agent implementation guidance and illustrations in ASU 2014-09; ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies identifying performance obligation and licensing implementation guidance and illustrations in ASU 2014-09; and ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which addresses implementation issues and is intended to reduce the cost and complexity of applying the new revenue standard in ASU 2014-09 (collectively, the “Revenue ASUs”). The Revenue ASUs noted above provide an accounting standard for a single comprehensive model for use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance. The accounting standard is effective for interim and annual periods beginning after December 15, 2017, with an option to early adopt for interim and annual periods beginning after December 15, 2016. The guidance permits two methods of adoption: retrospectively to each prior reporting period presented (the full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method). The Company currently anticipates adopting the new standard effective January 1, 2018 under the modified retrospective method. The Company has been monitoring FASB activity related to the new standard, as well as working with various non-authoritative groups to conclude on specific interpretative issues. The Company has established an implementation team to assess the impact of the standard on revenue recognition by reviewing our current accounting policies and practices to identify potential differences resulting from the application of the requirements of the new standard. The Company has identified and are in the process of implementing appropriate changes to our business controls, processes, and systems to support recognition and disclosure under the Revenue ASUs. In addition, the Company continues to monitor additional changes, modifications, clarifications or interpretations undertaken by the FASB, which may impact its conclusions. While the Company is substantially through our assessment of the new standard, the Company is still in the process of finalizing our implementation and determining the cumulative impact. The Company expects that under the new standard, the Company will continue to recognize revenue allocated to material rights to acquire certain licenses at a point in time when our collaboration partners exercise applicable options, which is consistent with our current revenue recognition model. The Company expects variable consideration from payments to R&D services will be allocable specifically to R&D services and will be recognized as earned under the new standard, whereas currently a portion of these payments are allocated to other undelivered elements, which could have a material impact on our financial statements. The impact will be recognized with a cumulative effect adjustment to equity at the date of initial application. The Company will adopt ASU 2014-09 effective January 1, 2018 using the modified retrospective transition method. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (Topic 718) (“ASU 2016-09”). The guidance changes how companies account for certain aspects of equity-based payments to employees. Entities will be required to recognize income tax effects of awards in the income statement when the awards vest or are settled. The guidance also allows an employer to repurchase more of an employee’s shares than it can under current guidance for tax withholding purposes providing for withholding at the employee’s maximum rate as opposed to the minimum rate without triggering liability accounting and to make a policy election to account for forfeitures as they occur. The Company adopted the new standard January 1, 2017. The Company made an accounting policy election to account for the impact of pre-vesting forfeitures as they occur rather than applying an estimated forfeiture rate, as previously required. Adoption did not materially impact the consolidated financial statements. In October 2016, the FASB issued ASU No. 2016-16, Income Taxes Intra-Entity Transfers of Assets Other Than Inventory In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In January 2017, the FASB issued ASU No. 2017-01, Business Combinations In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (“Topic 718”): Scope Modification Accounting. The new standard is intended to reduce the diversity in practice and cost and complexity when applying the guidance in Topic 718 to a change to the terms or conditions of a share-based payment award. The new standard will be effective beginning January 1, 2019.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, net	3. Property and Equipment, net Property and equipment, net, consists of the following (in thousands): As of December 31, 2017 2016 Computer equipment $ 285 $ 110 Furniture, fixtures, and other 2,104 2,044 Laboratory equipment 6,603 2,970 Leasehold improvements 13,776 15,780 Construction work in process — 1,065 22,768 21,969 Accumulated Depreciation (3,911 ) (942 ) Property and equipment, net $ 18,857 $ 21,027 Depreciation expense for the year ended December 31, 2017, 2016, and 2015 was $3.0 million, $0.9 million, and $0.1 million, respectively.

Variable Interest Entities

Variable Interest Entities	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Variable Interest Entities	4. Variable Interest Entities TRACR Hematology Limited On January 23, 2014, the founders of the Company formed TRACR in the United Kingdom, to further the development of the CRISPR/Cas9 technology into medicines for the treatment of blood-borne illnesses. On April 14, 2014, TRACR licensed certain foundational intellectual property rights under joint ownership from Dr. Emmanuelle Charpentier to develop and commercialize products for the treatment or prevention of human diseases related to hemoglobinopathies. See Note 9 for further details of the technology license agreement with Dr. Charpentier. On April 14, 2014 the Company determined that it became the primary beneficiary of TRACR based on, among other factors, the Company’s power to direct the activities that significantly impacted the economic performance of TRACR and the Company’s financing of contractual obligations on behalf of TRACR, and the period in which the Company began to benefit from research and development of TRACR technology. Accordingly, the Company consolidated TRACR’s financial statements as a consolidated VIE beginning on April 14, 2014. On March 24, 2015, the Company acquired 4,600 ordinary shares of TRACR, representing 82.1% of the ordinary share capital, pursuant to a share exchange transaction with the shareholders of TRACR. Pursuant to the share exchange transaction on March 24, 2015, the Company also entered into a freestanding call option agreement with Dr. Charpentier for 1,000 ordinary shares of TRACR, representing the remaining 17.9% of the ordinary share capital of TRACR. Under the terms of the call option agreement, the Company has the option to acquire the remaining 1,000 shares of TRACR held by Dr. Charpentier in exchange for 328,017 Common Shares of the Company. Upon IPO in October 2016, the call option was exercised and the remaining non-controlling interest of TRACR was acquired, resulting in a reduction of Noncontrolling interest of $0.1 million, stock-based compensation of $0.2 million for original value of the call option, and additional paid-in capital of $0.1 million and TRACR became a wholly-owned subsidiary of the Company. Joint Venture with Bayer Healthcare LLC In December 2015, the Company entered into an agreement with Bayer to create a joint venture to discover, develop and commercialize new therapeutics for genetically linked diseases, including blood disorders, blindness and heart disease. On February 12, 2016, Casebia, a limited liability partnership, was formed in the United Kingdom. In March 2016 upon consummation of the JV, Bayer and the Company each received a 50% equity interest in the entity in exchange for their contributions to the entity. The Company determined that Casebia was considered a VIE and concluded that it is not the primary beneficiary of the VIE. As such, the Company did not consolidate Casebia’s results into the consolidated financial statements. See Note 9 for further details.

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Intangible Assets	5. Intangible Assets The Company’s intangible assets consist of acquired intellectual property rights related to the Company’s initial consolidation of TRACR in April 2014. Acquired intellectual property rights had an estimated life of 10 years. Intangible assets, net of accumulated amortization, are as follows (in thousands): Acquired intangible asset Cost Accumulated Amortization Net As of December 31, 2017 $ 547 $ (203 ) $ 344 As of December 31, 2016 $ 547 $ (148 ) $ 399 The Company recorded amortization expense of $0.1 million, $0.1 million, and $0.1 million for each of the years ended December 31, 2017, 2016, and 2015, respectively. As of December 31, 2017 and 2016, the remaining amortization period was 6.3 years and 7.3 years, respectively. The Company has not recorded any impairment charges for the years ended December 31, 2017, 2016 and 2015. The estimated future amortization of acquired intangible assets as of December 31, 2017 is expected to be as follows (in thousands): For the Year Ended December 31, Amount 2018 $ 55 2019 55 2020 55 2021 55 2022 55 Thereafter 69 Total amortization $ 344

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses	6. Accrued Expenses Accrued expenses consist of the following (in thousands): As of December 31, 2017 2016 Payroll and employee-related costs $ 5,550 $ 2,585 Research costs 2,285 996 Licensing fees 609 492 Professional fees 2,176 2,715 Intellectual property costs 500 3,372 Accrued property and equipment — 5,081 Other 241 1,079 Total $ 11,361 $ 16,320

Convertible Loans

Convertible Loans	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Convertible Loans	7. Convertible Loans 2015 Convertible Loan Agreement with Vertex and certain existing shareholders On October 26, 2015, the Company entered into a convertible loan agreement with Vertex and certain existing shareholders (the “Vertex Convertible Loan”) under which the Company could borrow up to $40.0 million. The Vertex Convertible Loan accrues interest at 2.5% per annum and had an initial maturity date of April 26, 2016 subject to acceleration upon the occurrence of certain conditions stated in the loan agreement (the “Maturity Date”). On various dates between November 23 and December 7, 2015, the Company borrowed aggregate net proceeds of $38.2 million. The Vertex Convertible Loan included various embedded conversion, redemption and other features, as further described below, none of which required separate accounting from the host instrument under ASC 815. On January 29, 2016, all of the outstanding principal plus accrued interest of $0.2 million under the Vertex Convertible Loan was automatically converted into 2,859,278 Series B Preferred Shares in connection with a qualified financing described below. An event of default (“Event of Default”) is defined in the Vertex Convertible Loan Agreement and includes events of bankruptcy, insolvency or reorganization and, solely at the election of Vertex, a material breach that is not cured within the applicable notice and cure periods of the strategic collaboration, option and license agreement entered into by Vertex and the Company. See Note 9 for further details of the strategic, option and license agreement. Conversion Terms On the Maturity Date, the outstanding principal plus accrued interest automatically converts into Series B Preferred Shares at $9.33 per share. In the event the Company issues equity securities prior to the Maturity Date with aggregate proceeds of not less than $50.0 million, of which $5.0 million is raised from investors other than Vertex or existing shareholders, the outstanding principal plus accrued interest under the Vertex Convertible Loan automatically converts into the newly issued equity securities at the price per share paid by the investors in the financing. In the event of an underwritten public offering with shares of the Company listed on the New York Stock Exchange, the Nasdaq Global Market, or the Nasdaq Global Market, resulting in at least $50.0 million of proceeds to the Company closed prior to Maturity, the holders may elect, prior to the closing of the IPO, to convert the outstanding principal plus accrued interest into Series B Preferred Shares at $9.33 per share. Any Vertex Convertible Loan not converted prior to the closing of the IPO, shall automatically convert into Common Shares at a price paid by the investors for such shares in the IPO. Upon a liquidation event prior to the Maturity Date, the holders may elect to convert the outstanding principal plus accrued interest into either Common Shares at a price of $9.33 per share or Series B Preferred Shares at a price of $9.33 per share. Redemption Terms Upon an Event of Default, all outstanding principal plus accrued interest becomes immediately due and payable. Upon a liquidation event, if the holders do not exercise their conversion right, the outstanding principal plus accrued interest shall become due and payable in cash on the business day following the date on which the Company or its shareholders receive the proceeds from the liquidation event. Contingent Interest Upon an Event of Default, the outstanding amount of the Vertex Convertible Loan shall bear, in addition to the base interest of 2.5% per annum, default interest at a rate of 7.5% per annum. Convertible Loan with Bayer HealthCare LLC Concurrent with the execution of the Bayer Joint Venture agreement, the Company also entered into a Convertible Loan Agreement (“Bayer Convertible Loan”) with Bayer for $35.0 million. The Bayer Convertible Loan accrued interest at 2.0% per annum and matured on January 29, 2016 (the “Maturity Date”). On January 29, 2016, the Company issued the Bayer Convertible Loan in exchange for aggregate net proceeds of $35.0 million. The Bayer Convertible Loan included various embedded conversion, redemption and other features, none of which required separate accounting from the host instrument under ASC 815. Conversion of Convertible Loans to Series B Preferred Shares On January 29, 2016, concurrent with the issuance of the Bayer Convertible Loan, all of the outstanding principal under the $35.0 million Bayer Convertible Loan automatically converted into 2,605,330 Series B Preferred Shares at $13.43 per share. The Company determined the fair value of the Bayer Convertible Loan to be $24.5 million based on the fair value of the underlying Series B Preferred Shares that were exchanged as part of the immediate conversion. As the Bayer Convertible Loan was executed in contemplation of the joint venture agreement with Bayer, the Company evaluated the Bayer Convertible Loan as part of one multiple-element arrangement and using a relative fair value allocation allocated $27.0 million of aggregate arrangement consideration to the Bayer Convertible Loan upon issuance (See Note 9). Upon conversion, the Company accreted the Bayer Convertible Loan to its face value of $35.0 million through a charge to interest expense of $8.0 million and converted the $35.0 million to Series B Preferred Shares under the conversion model. The receipt of $35.0 million in proceeds under the Bayer Convertible Loan in exchange for equity securities, combined with the $38.2 million in proceeds from Vertex Convertible Loan, represented a qualified financing, as defined, and triggered an automatic conversion provision of the Vertex Convertible Loan Agreement. Accordingly, on January 29, 2016, the Vertex Convertible Loan, including loans from existing shareholders, plus accrued interest also converted into 2,859,278 of Series B Preferred Shares at $13.43 per share. The Company determined the fair value of the Vertex Convertible Loan to be $26.9 million based on the fair value of the underlying Series B Preferred Shares that were exchanged as part of the conversion. Upon extinguishment, the Company recorded a gain on extinguishment of $11.5 million for the difference between the carrying value of the debt and the fair value of the Series B Preferred Shares issued to settle the debt under the general extinguishment model.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Operating Leases As of December 31, 2017, the Company had operating In May 2016, the Company entered into a sublease pursuant to which it subleases in Cambridge, Massachusetts (the “610 Main Street Sublease”) the Company’s primary research and US office facility. The initial term of the 610 Main Street will expire on December 22, 2026. The Company has an option to extend the term of the 610 Main Street Sublease for an additional five year period if, at the time of expiration of the initial term, the sublessor does not intend to utilize the space for itself or its affiliates. The 610 Main Street Sublease contains escalating rent clauses which require higher rent payments in future years. In March 2017, the Company entered into an agreement pursuant to which it has the right to use certain office facilities in Zug, Switzerland. The original term expires in September 2017 with an automatic renewal for a six-month term. The Company’s obligations under this right to use agreement are secured by a cash deposit in the approximate amount of CHF 11 thousand held by the office space provider. Future minimum payments required under the leases as of December 31, 2017, are as follows (in thousands): Years Ended December 31, Amount 2018 $ 4,908 2019 6,369 2020 6,823 2021 7,027 2022 5,875 Thereafter 24,494 Total minimum lease payments $ 55,496 The amounts above are net of payments to be received under a sublease agreement, totaling $1.5 million in 2018 and $0.3 million in 2019. Letters of Credit As of December 31, 2017 and 2016, the Company had restricted cash of $3.2 million and $3.2 million, respectively, representing letters of credit securing the Company’s obligations under certain leased facilities in Cambridge, Massachusetts at 200 Sidney Street and the 610 Main Street as well as certain credit card arrangements. The letters of credit are secured by cash held in a restricted depository account. The cash deposit is recorded in restricted cash in the accompanying consolidated balance sheet as of December 31, 2017 and 2016. Shareholder Settlement Under the terms of a shareholder agreement existing prior to the IPO in 2016, if a U.S. common shareholder elected to file a Qualified Electing Fund (“QEF”) and notified the Company of this election, the Company was required to make advance payments to the shareholder related to their individual tax liability. In September 2016, the Company formally offered an aggregate settlement of up to $2.0 million to certain U.S common shareholders in order to release the Company from any and all obligations or claims concerning and/or arising out of the Company’s status as a PFIC or a Controlled Foreign Corporation (a “CFC”) for any taxable year from 2013 through 2015, including for potential lack of timely notification of the Company’s PFIC status (an “Annual Information Statement”) for the year ended December 31, 2015. Following the formal settlement offer in September 2016, in the fourth quarter of 2016 the Company made payments to shareholders of $2.0 million, respectively, under the terms of the accepted settlements. The obligation to make advance payments under the shareholder agreement for tax years subsequent to 2015 terminated upon the closing of the IPO. The Company has made available a 2017 and 2016 PFIC Annual Information Statement on its website for its shareholders. Research Agreements The Company has engaged several research institutions and companies to identify new delivery strategies and applications of the CRISPR/Cas9 technology. As a result of these efforts, the Company sponsored research programs during 2017. In association with these agreements, the Company has committed to making payments for related research and development services of $0.3 million, and $0.1 million in 2018 and 2019, respectively. The Company is also a party to a number of research license agreements which require upfront payments, future royalty payments and potential milestone payments from time to time which could be significant. In association with these agreements, the Company has committed to making payments for related research and development services of $0.5 million, and $0.4 million in 2018 and 2019, respectively. Litigation The Company licenses a U.S. patent application that is currently subject to interference proceedings declared by the Patent Trial and Appeal Board (“PTAB”) of the U.S. Patent and Trademark Office. Following motions by the parties and other procedural matters, the PTAB concluded in February 2017 that the declared interference should be dismissed because the claim sets of the two parties were not directed to the same patentable invention in accordance with the PTAB’s two-way test for patent interferences. In April 2017, the regents of the University of California (“California”) appealed the PTAB decision to the U.S. Court of Appeals for the Federal Circuit. In the appeal, California is seeking review and reversal of the PTAB’s February 2017 decision, which terminated the interference without determining which inventors actually invented the use of the CRISPR/Cas9 genome editing technology in eukaryotic cells.

Significant Contracts

Significant Contracts	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant Contracts	9. Significant Contracts Intellectual Property Agreements CRISPR Therapeutics AG—Charpentier License Agreement In April 2014, the Company entered into a technology license agreement with Dr. Emmanuelle Charpentier pursuant to which the Company licensed certain intellectual property rights under joint ownership from Dr. Charpentier to develop and commercialize products for the treatment or prevention of human diseases other than hemoglobinopathies (“CRISPR—Charpentier License Agreement”). In consideration for the granting of the license, the Company paid Dr. Charpentier an upfront fee of CHF 0.1 million ($0.1 million) and agreed to pay an immaterial annual license maintenance fee if Dr. Charpentier is not otherwise engaged in a service arrangement with the Company. During the years ended December 31, 2017, 2016 and 2015, Dr. Charpentier has been in a consulting arrangement with the Company, as such, no annual payments have been made under this provision. Dr. Charpentier is entitled to receive nominal clinical milestone payments. The Company is also obligated to pay Dr. Charpentier a low single digit percentage of sublicensing payments received under any sublicense agreement with a third party. In addition, the Company is also obligated to pay to Dr. Charpentier a low single-digit percentage royalty based on annual net sales of licensed products and licensed services by the Company and its affiliates and sublicensees. During the years ended December 31, 2017, 2016, and 2015 the Company recorded and accrued $0 million, $0.5 million, and $0.9 million, respectively, of sublicensing fees due to Dr. Emmanuelle Charpentier in research and development expense under the terms of the CRISPR—Charpentier License Agreement that was triggered by the execution of the Vertex collaboration agreement and the Bayer agreement. TRACR Hematology Limited—Charpentier License Agreement In April 2014, TRACR entered into a technology license agreement (“TRACR—Charpentier License Agreement”) with Dr. Emmanuelle Charpentier pursuant to which TRACR licensed certain intellectual property rights under joint ownership from Dr. Charpentier to develop and commercialize products for the treatment or prevention of human diseases related to hemoglobinopathies. In consideration for the granting of the license, Dr. Charpentier is entitled to receive nominal clinical milestone payments. TRACR is also obligated to pay Dr. Charpentier a low single digit percentage of sublicensing payments received under any sublicense agreement with a third party. In addition, TRACR is obligated to pay to Dr. Charpentier low single digit percentage royalties based on annual net sales of licensed products and licensed services by the Company and its affiliates and sublicensees. During each of the years ended December 31, 2017, 2016, and 2015 the Company recorded an immaterial amount of sublicensing fees due to Dr. Emmanuelle Charpentier in research and development expense under the terms of the TRACR—Charpentier License Agreement that was triggered by the execution of the Vertex collaboration agreements. Invention Management Agreement On December 15, 2016, the Company entered into an IMA, with the University of California (“California”), the University of Vienna (“Vienna”), Dr. Charpentier, Intellia therapeutics, Inc. (“Intellia”), Caribou Biosciences, Inc. (“Caribou”), ERS Genomics Ltd., or (“ERS”), and TRACR. Under the IMA, California and Vienna retroactively consent to Dr. Charpentier’s licensing of her rights to the CRISPR/Cas9 intellectual property, pursuant to the license the Company has with Dr. Charpentier, to the Company, and wholly-owned subsidiary TRACR, and ERS, in the United States and globally. The IMA also provides retroactive consent of co-owners to sublicenses granted by us, TRACR and other licensees, prospective consent to sublicenses they may grant in future, retroactive approval of prior assignments by certain parties, and provides for, among other things, (i) good faith cooperation among the parties regarding patent maintenance, defense and prosecution, (ii) cost-sharing arrangements, and (iii) notice of and coordination in the event of third-party infringement of the subject patents and with respect to certain adverse claimants of the CRISPR/Cas9 intellectual property. Unless earlier terminated by the parties, the IMA will continue in effect until the later of the last expiration date of the patents underlying the CRISPR/Cas9 technology, or the date on which the last underlying patent application is abandoned. Under the IMA the Company is obligated to share costs related to patent maintenance, defense and prosecution. For the years ended December 31, 2017, 2016, and 2015 the Company incurred $1.2 million, $2.8 million and $1.5 million respectively, in shared costs. The Company had accrued legal costs from the cost sharing of $0.4 million and $2.8 million as of December 31, 2017 and December 31, 2016, respectively. Patent Assignment Agreement In November 2014, the Company entered into a patent assignment agreement (“Patent Assignment Agreement”) with Dr. Emmanuelle Charpentier, Dr. Ines Fonfara, and Vienna (collectively, the “Assignors”), pursuant to which the Company was assigned all rights, title and interest in and to certain patent rights claimed in the U.S. Patent Application No.61/905,835. In consideration for the assignment of such rights, the Assignors are entitled to receive clinical milestone payments totaling up to €0.3 million (approximately $0.4 million) in the aggregate for the first human therapeutic product. The Company is also obligated to pay to the Assignors low single digit royalties based on annual net sales of licensed products and licensed services by the Company and its affiliates and sublicensees. During the years ended December 31, 2017, 2016, and 2015 the Company recorded $0 million, $33 thousand, and $0.1 million, respectively, of sublicensing fees due to the Assignors in research and development expense under the terms of the Patent Assignment Agreement that was triggered by the execution of the Vertex collaboration agreement and the Bayer Agreement. Collaboration Agreement and JDA with Vertex Pharmaceuticals, Incorporated Summary of Agreement On October 26, 2015, the Company entered into a strategic collaboration, option, and license agreement (“Collaboration Agreement”) with Vertex, focused on the use of CRISPR’s gene editing technology, known as CRISPR/Cas9, to discover and develop potential new treatments aimed at the underlying genetic causes of human disease. On December 12, 2017, the Company and Vertex entered into Amendment No. 1 to the Collaboration Agreement, or the (the “ Amendment”). The Amendment, among other things, modified certain definitions and provisions of the Collaboration Agreement to make them consistent with the JDA and clarified how many options are exercised (or deemed exercised) in connection with certain targets specified under the Collaboration Agreement. The Amendment also amended other provisions of the Collaboration Agreement, including the expiration terms of the Collaboration Agreement. Summary of Collaboration Agreement The collaboration will evaluate the use of CRISPR-Cas9 across multiple diseases where targets have been validated through human genetics. Vertex and CRISPR will focus their initial gene editing research on discovering treatments to address the mutations and genes known to cause and contribute to sickle cell disease (“SCD”), beta-thalassemia and cystic fibrosis. Vertex and CRISPR will also evaluate a specified number of other genetic targets as part of the collaboration. For up to six targets, Vertex has an exclusive option to obtain: (1) an exclusive license to commercialize CRISPR technology or (2) a co-exclusive license with respect to hemoglobinopathy and beta-globin targets. 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 (“R&D Services”) are related to generating genome editing reagents that modify gene targets selected by Vertex. Except with respect to the Company’s obligations under the mutually agreed upon research plan, Vertex has sole responsibility, at its own costs, for the worldwide research, development, manufacturing and commercialization of products resulting from the exclusive licenses obtained. The research collaboration will end on the earlier of the date on which Vertex has exercised six options to obtain exclusive/co-exclusive licenses with respect to a collaboration target, or the fourth anniversary of the effective date of the agreement. The research term may be extended as mutually agreed by the parties up to nine additional months to complete any research activities under the approved research plan that are incomplete on the fourth anniversary of the effective date. The Collaboration Agreement will be managed on an overall basis by a project leader from each of the Company and Vertex. In addition, the activities under the collaboration agreement during the research term will be governed by a joint research committee (“JRC”) formed by an equal number of representatives from the Company and Vertex. Decisions by the JRC will be made by consensus of the group, however, Vertex will have final decision-making authority in the event of disagreement, provided it is in good faith and not contrary to any explicit clause of the agreement. In connection with the Collaboration Agreement, Vertex made a nonrefundable upfront payment of $75.0 million. In addition, Vertex will fund all of the discovery activities conducted pursuant to the agreement. For potential hemoglobinopathy treatments, including treatments for sickle cell disease, the Company and Vertex will share equally all research and development costs and worldwide revenues. For other targets that Vertex elects to license, Vertex would lead all development and global commercialization activities. For each of up to six targets that Vertex elects to license, other than hemoglobinopathy and beta-globin targets, the Company has the potential to receive up to $420.0 million in development, regulatory and commercial milestones and royalties on net product sale. Vertex is entitled to terminate the Collaboration Agreement as a whole, or terminate the Collaboration Agreement in part with respect to a particular collaboration program, for convenience by providing the Company 90 days’ written notice of such termination; provided, however, that if any termination applies to a product for which Vertex has received marketing approval, Vertex will provide CRISPR no less than 270 days’ notice of such termination. If Vertex is in material breach of this Collaboration Agreement, the Company has the right to terminate the Collaboration Agreement in full at its discretion 90 days after delivery of written notice to Vertex. Summary of Amendment and JDA Under the terms of the Collaboration Agreement, Vertex was given a total of six target options available to be exercised. Upon execution of the JDA and Amendment, Vertex exercised two of its original six options to develop and commercialize hemoglobinopathy and beta-globin targets. Under the terms of the JDA and Amendment, inclusion of further hemoglobinopathy targets will require mutual agreement of the Parties and will not require Vertex to exercise additional options. In connection with entering into the JDA, the Company received a $7.0 million up-front payment from Vertex and is eligible for a one-time low seven-digit milestone payment upon the dosing of the second patient in a clinical trial with the initial product candidate. The net profits and net losses, as applicable, incurred under the JDA will be shared equally between us and Vertex. CRISPR and Vertex will form the following committees: (i) a joint steering committee to provide high-level oversight and decision making regarding the activities covered by the JDA, (ii) a joint development committee to provide oversight and decision making-making regarding development activities, (iii) a joint commercialization committee to provide oversight and decision-making regarding commercialization activities and (iv) a joint manufacturing committee to provide oversight and decision-making regarding manufacturing activities. Each of the committees will contain an equal number of representatives from each of CRISPR and Vertex. The JDA provides that the Company will be responsible for commercialization activities in the United States and Vertex will be responsible for commercialization activities outside of the United States. Either party can terminate the JDA upon the other party’s material breach, subject to specified notice and cure provisions, or, in the case of Vertex, in the event that the Company becomes subject to specified bankruptcy, winding up or similar circumstances. Either party may terminate the JDA in the event the other party commences or participates in any action or proceeding challenging the validity or enforceability of any patent that is licensed to such challenging party pursuant to the JDA. Vertex also has the right to terminate the JDA for convenience at any time after giving prior written notice. If circumstances arise pursuant to which a party would have the right to terminate the JDA on account of an uncured material breach, such party may elect to keep the JDA in effect and cause such breaching party to be treated as if it had exercised its opt-out rights with respect to the products associated with such uncured material breach (described below) and the royalties payable to the breaching party would be reduced by a specified percentage. Either party may opt of out of the development of a product candidate under the JDA after predetermined points in the development of the product candidate, on a candidate-by-candidate basis. In the event of such opt-out, the opting-out party will no longer share in the net profits and net losses associated with such product candidate and, instead, the opting out party will be entitled to high single to mid- teen percentage royalties on the net sales of such product, if commercialized. Vertex shall have no obligation to pay the Company any milestone payments with respective to an opt-out product. Accounting for the Collaboration Agreement, Amendment and JDA The Company determined that the Amendment and JDA represented a modification to the original Collaboration Agreement. As a result, the Company evaluated all remaining units of account within the modified contract and allocate to those units (based on their relative selling prices) the sum of (1) additional consideration related to the Amendment and JDA and (2) any deferred revenue from the Collaboration Agreement using current estimated prices for each of the units. The arrangement includes components of a customer-vendor relationship and a collaborative arrangement as defined under ASC 808. The Company will apply the guidance of ASC 605-25 by analogy to the units of account of the Collaboration Agreement and the units of account of the JDA subject to ASC 605-25 as outlined below. The Company will apply the guidance of ASC 808 to the on-going collaborative and development components of the JDA including the (i) development and commercialization services for currently identified shared products; (ii) R&D services for any follow-on products subject to the JDA; and (iii) committee participation. The Company evaluated the Collaboration Agreement, Amendment and JDA in accordance with the provisions of ASC 605-25. The Company’s arrangement with Vertex contains the following deliverables: (i) a non-exclusive research license; (ii) an option to obtain an exclusive license for up to four Collaboration Targets; (iii) co-exclusive development and commercialization licenses for hemoglobinopathy and beta-globin targets identified in the JDA; (iv) co-exclusive research license for the follow-on products; (v) the performance of R&D Services under the Collaboration Agreement; and (vi) JRC participation under the Collaboration Agreement. Management considered whether any of these deliverables could be considered separate units of accounting. Regarding the non-exclusive research license, the Company concluded that it does not have stand-alone value separate from the options to exercise the exclusive or the exercised co-exclusive licenses since Vertex would not benefit from acquiring a research license without the ability to obtain the license to commercialize the results of that research. As a result, the Company concluded that the research license should be combined with those options. Regarding the co-exclusive research license for the follow-on products, the Company concluded that it does not have stand-alone value separate from the exercised co-exclusive licenses under the JDCA since Vertex would not benefit from acquiring a research license without the license to commercialize the results of that research. The Company concluded the co-exclusive research license should be combined separately with each development and commercialization co-exclusive license granted under the JDA. Regarding the R&D Services under the Collaboration Agreement, the Company concluded that there are other vendors in the market that could perform the related services. As such the Company concluded the R&D Services represent a separate unit of accounting. Regarding the JRC obligations, the Company concluded that the JRC obligations deliverable has standalone value from the option to license because the services could be performed by an outside party. As such the Company concluded the JRC obligations represent a separate unit of accounting. As a result, management concluded that there following the modification are four units of accounting: (i) four individual combined units of accounting representing the non-exclusive research license, and the option for up to four exclusive licenses to develop and commercialize the collaboration targets; (ii) four individual combined units of accounting representing the co-exclusive research license, and a development and commercialization license to develop and commercialize hemoglobinopathies and beta-globin targets; (iii) the performance of R&D Services; and (iv) the participation in the JRC. 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. 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 and JDA 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 and the JRC participation 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’s BESP for the remaining R&D Services was $4.0 million. The Company’s BESP for the JRC participation services was de minimis based on an estimate of time spent on preparation, participation, review and travel for the meetings. The Company’s BESP for the remaining four combined units of the non-exclusive research license and the options for an exclusive license to develop and commercialize a single collaboration target are $55.6 million, $48.4 million, $27.3 million and $27.3 million for a total of $158.6 million. BESPs for these items were determined based on probability and present value adjusted cash flows from the milestones payments owed for exclusive licenses outlined in the Collaboration Agreement. BESP reflects the level of risk and expected probability of success inherent in the nature of the associated research area. The Company’s BESP for the co-exclusive research license and the development and commercialization licenses for of the hemoglobinopathy and beta-globin targets is $48.9 million. BESP for this item was determined based on probability and present value adjusted cash flows from the equal sharing of project worldwide net profit or net loss. BESP reflects the level of risk and expected probability of success inherent in the nature of the associated research area. Allocable arrangement consideration is comprised of: (i) deferred revenue of $80.0 million; (ii) an upfront payment of $7.0 million under the JDA; (iii) the estimated R&D services of $4.0 million; and (iv) payments related to the estimated exercise of options on future exclusive licenses for four targets of $40.0 million. The aggregate allocable arrangement consideration of $131.0 million was allocated among the separate units of accounting using the relative selling price method as follows: (i) four individual combined units of accounting representing the non-exclusive research license, and the option for up to four exclusive licenses to develop and commercialize the collaboration targets: $34.4 million, $30.0 million, $16.9 million and $16.9 million for a total of $98.2 million; (ii) four individual combined units of accounting representing the co-exclusive research license, and a development and commercialization license to develop and commercialize each of the hemoglobinopathy and beta-globin targets: $30.3 million; and (iii) the performance of R&D Services: $2.5 million. Regarding each of the four individual combined units of accounting representing the non-exclusive research license, and the option for up to four exclusive licenses to develop and commercialize the collaboration targets, the Company will defer recognition of the amount attributable to each unit of accounting until the respective option is exercise. If Vertex ultimately fails to exercise any of the remaining license options, any remaining amount allocated to the combined unit of account would be recognized when the Company’s applicable option expires, provided that revenue recognized will be limited to cash received under the arrangement. Upon the execution of the JDA, a research, development and commercialization license has been granted for hemoglobinopathy and beta-globin targets. The Company determined that these licenses were delivered at inception of the JDA and $30.3 million in revenue was recognized for this unit of accounting in December 2017. Consistent with the Company’s original accounting policy, the amounts allocated to R&D Services will be recognized as the R&D Services are performed. Milestones under the Collaboration Agreement The Company has evaluated all of the milestones that may be received in connection with the Collaboration Agreement. In evaluating if a milestone is substantive, the Company assesses whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance, and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company notes that the $10.0 million due upon the exercise of each option for an Exclusive License was determined to be part of the fixed and determinable consideration allocable at contract inception and is not subject to milestone method accounting. The first potential milestone the Company will be entitled to receive is the milestone in the JDA to receive a one-time low seven-digit milestone payment in any clinical trial in the initial shared product. The Company determined that this payment represents the definition of a milestone under ASC 605-28 as: 1) dosing a patient in the initial shared product can only be achieved partially through CRISPR’s performance as they are co-developing the product, 2) while it is likely that this event will occur, there is uncertainty that the event will be achieved given the nature of clinical development and the stage of the CRISPR technology and 3) achievement of this milestone will result in an additional low seven- digit milestone payment being due to the Company. Additionally, the $10.0 million milestone due upon the filing of an Investigational New Drug Application (“IND”) for a selected Exclusive License under the Collaboration Agreement. As the developmental milestone of the agreement relates to the filing of an IND, the Company has considered it to be substantive. 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. There are no other substantive milestones. As such the total amount of substantive milestones subject to milestone method accounting treatment is $10.0 million for each selected Exclusive License and a one-time low seven-digit milestone payment for the JDA upon dosing of the second patient in any clinical trial in the initial shared product. The remaining milestones are predominately related to the development and commercialization of a product resulting from the arrangement and are payable with respect to each selected Exclusive License. Each milestone is payable only once per collaboration target, regardless of the number of products directed to such collaboration target that achieve the relevant milestone event. There are nine remaining clinical development and regulatory approval milestones which may trigger proceeds of up to $90.0 million and $235.0 million, respectively, for each selected Exclusive License, and two commercial milestones which may trigger proceeds of up to $75.0 million for each selected Exclusive License (which, when combined with the $10.0 million due upon exercise of the exclusive option and the $10.0 million development milestone associated with an IND, total $420.0 million for each selected Exclusive License), as follows: Developmental Milestone Events 1. Initiation of the first Clinical Trial of a Product 2. Establishment of POC for a Product 3. Initiation of the first Phase 3 Clinical Trial of a Product 4. Acceptance of Approval Application by the FDA for a Product 5. Acceptance of Approval Application by the EMA for a Product 6. Acceptance of Approval Application by a Regulatory Authority in Japan for a Product 7. Marketing Approval in the US for a Product 8. Marketing Approval in the EU for a Product 9. Marketing Approval in Japan for a Product Commercial Milestone Events 1. Annual Net Sales for Products with respect to a Collaboration Target exceed $500 million 2. Annual Net Sales for Products with respect to a Collaboration Target exceed $1.0 billion After Vertex has exercised an Exclusive License option, Vertex will be solely responsible for all research, development, manufacturing, and commercialization of licensed agents and products for the relevant target. As the Company’s involvement in this process is limited to observer status, management determined that milestones are not considered substantive because they do not relate solely to the past performance of the Company. Upon the achievement of a milestone, management will evaluate whether the triggering event occurs during or after the research term. If the triggering event occurs during the research term, management has elected to treat the milestone similar to an up-front payment. In these cases, if and when any of these milestones are received, the amount will be included in the overall arrangement consideration and allocated to the remaining identified deliverables. To the extent all deliverables have been satisfied, any additional consideration allocated to them could be immediately recognized. If the triggering event occurs after the research term, the Company will recognize the associated revenue in the period in which the event occurs. The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met. During the year ended December 31, 2017, 2016, and 2015, the Company recognized $36.2 million, $4.0 million, and $0.2 million of revenue with respect to the collaboration with Vertex. Research and development expense incurred by the Company in relation to its performance under the collaboration agreement for the years ended December 31, 2017 and 2016 was $9.9 million and $7.0 million, respectively. As of December 31, 2017 and 2016, there is $56.8 million and $77.1 million of non-current deferred revenue related to the Company’s collaboration with Vertex, respectively. Joint Venture with Bayer Healthcare LLC On December 19, 2015, the Company entered into an agreement to establish a joint venture (“Bayer Joint Venture”) to research the development of new therapeutics to cure blood disorders, blindness, and congenital heart disease. On February 12, 2016, the Company and Bayer completed the formation of the joint venture entity, Casebia, a limited liability partnership formed in the United Kingdom. Bayer and the Company each received a 50% equity interest in the entity in exchange for their contributions to the entity. The Company contributed $0.1 million in cash and licensed its proprietary CRISPR/Cas9 gene editing technology and intellectual property for selected disease indications. Bayer contributed its protein engineering expertise and relevant disease know-how. Bayer will provide up to $300.0 million in research and development funding to Casebia over the first five years, subject to certain conditions, of which the first $45.0 million was contributed upon formation in the first quarter of 2016. Under the joint venture agreement, the Company has no obligation to provide any additional funding and the Company’s ownership interest will not be diluted from future contributions from Bayer. The activities of Casebia are controlled by a management board under the joint control of the Company and Bayer. As Casebia is jointly controlled by the Company and Bayer, the Company accounts for its 50% interest using the equity method of accounting. Under the agreement, Casebia will pay the Company up to $35.0 million in exchange for a worldwide, exclusive license to commercialize the Company’s CRISPR/Cas9 technology specifically for the indications designated by Casebia. In March 2016, the Company received a non-refundable up-front payment of $20.0 million as a technology access fee. The remaining $15.0 million was paid on December 22, 2016 following delivery of the necessary consents from patent holders of the Company’s intellectual property. There are no milestone, royalties or other payments due to the Company under this aspect of the agreement. The Company determined that the contribution of the CRISRP/Cas9 technology by license to Casebia did not meet the definition of a business under ASC 805. The Company will also provide to Casebia compensated research and development services through a separate agreement. Concurrent with the execution of the Bayer Joint Venture agreement, the Company also entered into the Bayer Convertible Loan for $35.0 million. As the Bayer Joint Venture (including the CRISPR/Cas9 technology license and the research and development services) and the Bayer Convertible Loan were executed at the same time, the Company determined that they should be evaluated as one multiple-element arrangement. Additionally, the Company also determined that ASC 845, Nonmonetary Transactions The Company allocated the fair value of the consideration received using a relative fair value allocation. The allocable arrangement consideration included (i) the total cash payment by Casebia for the technology access fee, net of the Company’s $0.1 million contribution, of $34.9 million, (ii) the fair value of the equity interest in the Joint Venture of $36.4 million, (iii) the $35.0 million received from the issuance of the Convertible Debt, and (iv) $6.3 million of estimated cash consideration to be received under the research and development service arrangement, accumulating to $112.6 million. The Company identified the following elements under the transaction: (i) Combined element of an exclusive, worldwide, royalty free, license t

Redeemable Convertible Preferre

Redeemable Convertible Preferred Shares	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Redeemable Convertible Preferred Shares	10. Redeemable Convertible Preferred Shares Upon the closing of the Company’s IPO on October 24, 2016, all outstanding Preferred Shares of the Company were automatically converted into 27,135,884 Common Shares on a one-for-one basis. As of December 31, 2017 and 2016, the Company had no Preferred Stock authorized, issued, or outstanding. As of December 31, 2015, the Company had 18,837,024 registered Preferred Shares issued and outstanding in share capital, which was comprised of (i) 440,001 Series A-1 Preferred Shares CHF 0.03 par value per share; (ii) 3,120,001 Series A-2 Preferred Shares, CHF 0.03 par value per share; (iii) 10,758,006 Series A-3 Preferred Shares, CHF 0.03 par value per share; and, (iv) 4,519,016 Series B Preferred Shares, CHF 0.03 par value per share, (collectively, the “Preferred Shares”) . The Company’s redeemable convertible preferred shares were classified as temporary or mezzanine equity on the accompanying consolidated balance sheets in accordance with authoritative guidance for the classification and measurement of redeemable securities as the Preferred Shares are contingently redeemable at the option of the holders. In April 2015, the Company issued 10,758,006 Series A-3 Preferred Shares in exchange for $4.24 per share whereby $2.12 per share was received upon issuance, resulting in gross proceeds of $22.8 million and the balance of $2.12 per share was due upon meeting certain milestones. As of December 31, 2015, none of the milestones had occurred and the Company had an outstanding subscription receivable of $22.8 million related to the Series A-3 Preferred Shares. In connection with the issuance of the Series A-3 Preferred Shares, the Company amended the dividend and conversion terms of the Series A-1 and Series A-2 Preferred Shares. The Company’s policy requires the evaluation of amendments to equity classified preferred shares qualitatively to determine whether they are considered a modification or extinguishment. Based on this approach, the amendment to the terms of the Series A-1 and A-2 Preferred Shares was considered a modification and as a result, there was no adjustment to the carrying value of the Series A-1 and A-2 Preferred Shares. The balance of the Series A-3 Preferred Share subscription receivable of $2.12 per share was called on May 5, 2016 by the Board of Directors and gross proceeds of $22.8 million were received by May 27, 2016. In May 2015, the Company issued 4,519,016 Series B Preferred Shares in exchange for CHF 6.20 ($6.74) per share resulting in gross proceeds of CHF 28.0 million ($30.5 million). In January 2016, the Company issued 5,464,608 Series B Preferred Shares upon conversion of $38.4 million of Vertex Convertible Loans plus accrued interest and $35.0 million of Bayer Convertible Loans at a conversion price of $13.43 per share. In June 2016, the Company issued 2,834,252 Series B Preferred Shares in exchange for $13.43 per share resulting in gross proceeds of $38.1 million.

Share Capital

Share Capital	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Share Capital	11. Share Capital The Company had 41,092,969 and 40,253,674 Conditional Capital Reserved for Future Issuance The Company had the following conditional capital reserved for future issuance: As of December 31, Type of Share Capital Conditional Capital 2017 2016 Common Shares Unvested unissued restricted stock 166,667 166,667 Common Shares Outstanding stock options 6,262,339 4,535,371 Common Shares Reserved for future issuance under stock option plans (1) 4,657,700 5,290,643 Common Shares Shares available for bonds and similar debt instruments 4,919,700 4,919,700 Common Shares Shares available for employee purchase plans 413,226 413,226 Total 16,419,632 15,325,607 (1) The Company’s shareholders approved an increase to the option pool of 2,012,684 in May 2017. Common Share Issuances In October 2016, the Company completed an IPO whereby the Company sold 4,429,311 of its Common Shares, inclusive of 429,311 Common Shares sold by the Company pursuant to the partial exercise of an overallotment option granted to the underwriters in connection with the offering. Concurrent with the IPO, the Company issued and sold 2,500,000 Common Shares to Bayer BV, in a private placement. Additionally, the Company issued and subsequently reacquired the unexercised overallotment Common Shares of 170,689 at no cost, which are held in treasury. In January 2018, the Company completed an offering of 5,750,000 shares of our common shares, which were sold at a price of $22.75 per share. This offering resulted in $122.6 million of net proceeds to the Company, refer to Note 17 “Subsequent Events,” in the accompanying notes to the consolidated financial statements for further details. The Common Shares have the following characteristics: Voting Rights The holders of Common Shares are entitled to one vote for each Common Share held at all meetings of shareholders and written actions in lieu of meetings. Dividends The holders of Common Shares are entitled to receive dividends, if and when declared by the Board of Directors. As of December 31, 2017, no dividends have been declared or paid since the Company’s inception. Liquidation The holders of the Common Shares are entitled to share ratably in the Company’s assets available for distribution to shareholders in the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or upon the occurrence of a deemed liquidation event.

Equity-based Compensation

Equity-based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity-based Compensation	12. Equity-based Compensation Option and Grant Plans In July 2016, the shareholders approved the 2016 Share Option and Incentive Plan (the “2016 Plan”) and in April 2015, the shareholders approved the 2015 option and grant plan (the “2015 Plan” collectively the “Plans”). Subsequent to the IPO, no further options shall be granted under the 2015 Plan. The Plans provide for the issuance of equity awards in the form of restricted shares, options to purchase Common Shares which may constitute incentive stock options (“ISOs”) or non-statutory stock options (“NSOs”), unrestricted stock unit grants, and qualified performance and market-based awards to eligible employees, officers, directors, non-employee consultants, and other key personnel. Terms of the equity awards, including vesting requirements, are determined by the Board, subject to the provisions of the Plans. Options granted by the Company typically vest over four years and have a contractual life of ten years. The Company’s Board of Directors approved an increase to the option pool of 2,012,684 options in May 2017. During the years ended December 31, 2016 and 2015, the Company also issued outstanding Common Shares previously held by Founders and Fay Corp. to employees and non-employees as equity-based compensation (“Founder Awards”), which are subject to repurchase by the Company upon termination of the holder’s service relationship with the Company as well as upon certain triggering events such as termination for cause, material breach of agreement and insolvency of the holder that generally lapse over a requisite service period of four years. Equity-Based Compensation Expense The Company uses the straight-line attribution method to recognize stock-based compensation expense for stock options and restricted stock awards. Stock options and restricted stock generally vests over four years with 25% vesting on the first anniversary, and the remaining vesting monthly thereafter. The following table presents stock-based compensation expense in the Company’s Consolidated Statements of Operations: Years Ended December 31, 2017 2016 2015 Research and development $ 8,800 $ 4,848 $ 1,924 General and administrative 10,073 5,844 1,760 Loss from equity method investment 1,763 152 — Total $ 20,636 $ 10,844 $ 3,684 Grant- Date Fair Value The Company estimated the fair value of each employee and non-employee stock option award on the grant date using the Black-Scholes option-pricing model based on the following assumptions: Years Ended December 31, 2017 2016 2015 Employees: Options granted 2,894,850 2,411,240 1,913,319 Weighted - average exercise price $ 16.92 $ 12.19 $ 2.32 Weighted-average grant date fair value $ 10.86 $ 8.47 $ 3.11 Assumptions: Weighted-average expected volatility 72.1 % 81.0 % 76.4 % Expected term (in years) 6.0 6.0 6.0 Weighted-average risk free interest rate 2.0 % 1.4 % 1.7 % Expected dividend yield 0.0 % 0.0 % 0.0 % Non employees: Options granted 104,997 215,710 26,667 Weighted- average exercise price $ 18.74 $ 19.54 $ 1.85 Weighted- average grant date fair value $ 19.35 $ 17.38 $ 5.05 Assumptions: Weighted average expected volatility 81.5 % 88.2 % 84.1 % Expected term (in years) 9.4 10.0 10.0 Weighted-average risk free interest rate 2.4 % 2.4 % 2.2 % Expected dividend yield 0.0 % 0.0 % 0.0 % The fair value of the restricted stock awards was determined based on the fair value of Common Stock on the grant date. Non-employee stock options and restricted stock awards are marked-to-market at each reporting period. Share Based Payment Activity Stock Options The following table summarizes stock option activity for employees and non-employees during the year ended December 31, 2017 (intrinsic value in thousands): Stock Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (years) Aggregate Intrinsic Value Outstanding at December 31, 2016 4,535,371 $ 8.38 9.1 $ 53,975 Granted 2,999,847 $ 16.98 Exercised (862,227 ) $ 3.53 Cancelled or forfeited (410,652 ) $ 7.06 Outstanding at December 31, 2017 6,262,339 $ 13.24 8.8 64,120 Exercisable at December 31, 2017 1,617,647 $ 9.64 8.3 $ 22,391 Vested or expected to vest at December 31, 2017 (1) 6,262,339 $ 13.24 8.8 $ 64,120 (1) This represents the number of vested stock options as of December 31, 2017 plus the unvested outstanding options at December 31, 2017 expected to vest in the future. The total unrecognized compensation cost for employee and non-employee stock options is adjusted for estimated forfeitures. As of December 31, 2017, the Company expects to recognize total unrecognized compensation cost related to stock options of $42.2 million over a remaining weighted-average period of 3.4 years. During 2017 and 2016, the Company granted options to purchase 60,000 and 123,333 Common Shares, respectively, subject to performance-based vesting conditions. As of December 31, 2017, options to purchase 362,872 Common Shares subject to performance-based vesting conditions were vested, as performance conditions were achieved, and there were no options to purchase Common Shares subject to performance-based vesting conditions outstanding. During 2017, the Company also granted 150,000 stock options with market-based vesting conditions in which the recipient is eligible to receive between zero and 150,000 options to purchase Common Shares at the end of a four year service period based upon achieving a specified average stock price. As of December 31, 2017, no options to purchase Common Shares subject to market-based vesting conditions were vested. During the year ended December 31, 2017, the Chairman of the Board of Directors resigned. Coinciding with his resignation, he and the Company entered into an advisory agreement to perform consulting services. In consideration for performing consulting services, he was allowed to continue to vest in the options during the advisory period, with a post-termination exercise period of 150 days from the last day of the service relationship or the expiration of the option, whichever is earlier. The service relationship is expected to continue until September 10, 2019. Management determined that his services to be provided under the advisory agreement were not substantive. Therefore, the entire $2.2 million of compensation expense to be recognized prospectively was recognized immediately. Restricted Stock The following table summarizes restricted stock activity for employees and non-employees during the year ended December 31, 2017: Reflected as outstanding upon vesting Reflected as outstanding upon grant date Total Weighted- Average Grant Date Fair Value Unvested restricted common shares at December 31, 2016 89,367 650,856 740,223 $ 3.84 Granted 101,667 — 101,667 $ 17.45 Vested (33,519 ) (418,032 ) (451,551 ) $ 6.40 Cancelled or forfeited — (23,938 ) (23,938 ) $ 1.72 Unvested restricted common shares at December 31, 2017 157,515 208,886 366,401 $ 8.49 During the years ended December 31, 2017 and 2016, the total fair value of restricted stock vested was $8.3 million and $9.9 million, respectively. At December 31, 2017, total unrecognized compensation expense related to unvested restricted stock was $3.6 million which the Company expects to recognize over a remaining weighted-average period of 1.3 years. During the year ended December 31, 2016, the Company and Fay Corp. transferred 290,400 Common Shares to a founder, 268,093 of which are subject to vesting conditions with a weighted average grant date fair value of $12.65 per share. The unvested Common Shares are subject to repurchase by the Company upon termination of the holder’s service relationship with the Company as well as upon certain triggering events such as termination for cause, material breach of agreement and insolvency of the holder. During the years ended December 31, 2017 and 2016, the Company recognized expense related to the Common Shares transferred to the Founder of $0.8 million and $2.6 million, respectively.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
401(k) Savings Plan	13. 401(k) Savings Plan The Company established a defined‑contribution savings plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”) in November 2016. 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. The Company contributed $0.5 million to the 401(k) Plan for the year ended December 31, 2017.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes The Company is subject to U.S. federal and various state corporate income taxes as well as taxes in foreign jurisdictions for the foreign parent and where foreign subsidiaries have been established. Net loss before taxes For the years ended December 31, 2017, 2016 and 2015, the loss before provision for income taxes consist of the following (in thousands): Years Ended December 31, 2017 2016 2015 Domestic $ 5,184 $ 3,322 $ 593 Foreign (71,792 ) (26,040 ) (26,414 ) Total $ (66,608 ) $ (22,718 ) $ (25,821 ) The (provision for) benefit from income taxes consist of the following (in thousands): Years Ended December 31, 2017 2016 2015 Current income taxes: Federal $ (1,533 ) $ (649 ) $ (23 ) State (42 ) 11 (12 ) Foreign 6 17 (26 ) Total current income taxes (1,569 ) (621 ) (61 ) Deferred income taxes: Federal (477 ) 30 (37 ) State 297 105 65 Foreign — 2 26 Total deferred income taxes (180 ) 137 54 Total income tax provision $ (1,749 ) $ (484 ) $ (7 ) A reconciliation of income tax expense computed at the statutory corporate income tax rate to the effective income tax rate for the years ended December 31, 2017, 2016 and 2015 is as follows: Years Ended December 31, 2017 2016 2015 Income tax expense at statutory rate 9.3 % 10.3 % 10.3 % State income tax, net of federal benefit 0.3 % 1.3 % 0.1 % Nondeductible expenses 0.0 % 1.6 % 0.0 % Foreign rate differential (2.5 %) (3.3 %) (1.4 %) Statutory to US GAAP permanent differences 1.8 % 6.6 % 0.0 % Stock-based compensation (2.9 %) (4.9 %) (1.4 %) Research credits 0.8 % 3.1 % 0.6 % Change in valuation allowance (9.4 %) (16.8 %) (8.2 %) Effective income tax rate (2.6 %) (2.1 %) — The federal statutory rate reflects the Switzerland mixed company service rate. Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets are comprised of the following (in thousands): Years Ended December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 9,987 $ 3,934 Accruals and reserves 1,123 791 Deferred Rent 3,494 5,228 Other deferred tax assets 36 7 Deferred revenue 1,721 2,525 Research credit 543 425 Total deferred tax assets 16,904 12,910 Less valuation allowance (13,041 ) (6,770 ) Net deferred tax assets 3,863 6,140 Deferred tax liabilities: Depreciation (3,791 ) (5,909 ) Intangible assets (59 ) (68 ) Other deferred tax liabilities (31 ) — Total deferred tax liabilities (3,881 ) (5,977 ) Long term deferred taxes $ (18 ) $ 163 The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses in its non-U.S. jurisdictions, the Company has concluded that it is more-likely-than-not that the benefit of its non-U.S. deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance against its net deferred tax assets in Switzerland, and in the UK for its TRACR subsidiary, as of December 31, 2017 and 2016. The valuation allowance increased by $6.3 million during 2017, which is primarily attributable to losses in Switzerland. Additionally, the Company has established a valuation allowance for certain U.S. deferred tax assets. On December 22, 2017, the Tax Cuts and Jobs Act ("the Act") was enacted in the United States. The Act reduces the U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allows the recording of provisional amounts during a measurement period not to extend beyond one year of the enactment date. The final impact may differ from this provisional amount due to, among other things, changes in interpretations and assumptions the Company has made thus far and the issuance of additional regulatory or other guidance. The Company expects to complete the final impact within the measurement period. As a result of the Act, the Company remeasured certain deferred tax assets and liabilities based on the rates at which they are anticipated to reverse in the future, which is generally 21%. This resulted in a decrease to the Company’s gross deferred tax assets of $0.2 million. The impact of the rate reduction on the Company’s deferred tax assets and liabilities is provisional. As of December 31, 2017, the Company had available non-U.S. net operating loss carryforwards of $127.4 million which begin to expire in 2020. As of December 31, 2017, the Company has U.S. domestic state research and development credit carryforwards of $0.4 million which begin to expire in 2032. As of December 31, 2017, the Company has U.S. domestic federal research and development credit carryforwards of $0.3 million which expire in 2037, which are net of uncertain tax positions of $0.2 million. ASC 740 clarifies the accounting for uncertainty in income taxes recognized in an enterprise’s financial statement by prescribing the minimum recognition threshold and measurement of a tax position taken or expected to be taken in a tax return. As of December 31, 2017 the Company had gross unrecognized tax benefits of $0.4 million of which $0.3 million would favorably impact the effective tax rate if recognized. The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2017, 2016 and 2015, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s consolidated statements of operations and comprehensive loss. The aggregate changes in gross unrecognized tax benefits was as follows (in thousands): Years Ended December 31, 2017 2016 2015 Balance at beginning of year $ 163 $ 49 $ — Increases for tax positions taken during current period 178 134 49 Increases for tax positions taken in prior periods 13 — — Decreases for tax positions taken during current period — — — Decreases for tax positions taken in prior periods — (20 ) — Balance at end of year $ 354 $ 163 $ 49 The Company files income tax returns in the U.S. federal jurisdiction, Massachusetts, and certain non-U.S. jurisdictions. The Company is subject to U.S. federal, Massachusetts, and non-U.S. income tax examinations by authorities for all tax years.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	15. Selected Quarterly Financial Data (Unaudited) Prior to its IPO on October 18, 2016, the Company had outstanding participating Preferred Shares. During the fourth quarter of the year ended December 31, 2016, the Company had net income, although for the full year the Company had a net loss. Accordingly, the Company used the two-class method to calculate net income per share for the fourth quarter of 2016. For purposes of calculating basic net income per share for the fourth quarter of 2016, the Company excluded from the numerator $3.1 million of net income attributable to participating securities. The Company calculated diluted net income per share under both the if-converted method and the two-class method and concluded that the two-class method was more dilutive than the if-converted method. Accordingly, the two-class income allocations were reapplied after taking into account the dilutive effect of non-participating securities. This resulted in net income of $3.1 million being allocated to the participating securities and excluded from the numerator of the Common Stock dilutive net income per share calculation. 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Collaboration revenue $ 2,703 $ 3,582 $ 2,387 $ 32,325 Total operating expenses 23,447 24,888 25,957 $ 31,353 (Loss) income from operations (20,744 ) (21,306 ) (23,570 ) $ 972 Net (loss) income (21,475 ) (22,315 ) (24,707 ) $ 140 Net (loss) income attributable to common shareholders (21,475 ) (22,315 ) (24,707 ) $ 140 Net (loss) income per share attributable to common shareholders: Basic $ (0.54 ) $ (0.56 ) $ (0.62 ) $ 0.00 Diluted $ (0.54 ) $ (0.56 ) $ (0.62 ) $ 0.00 Weighted-average common shares outstanding used in net (loss) income per share attributable to common shareholders: Basic 39,725,947 39,895,938 40,088,718 40,509,897 Diluted 39,725,947 39,895,938 40,088,718 41,635,843 2016 First Quarter Second Quarter Third Quarter Fourth Quarter (1) Collaboration revenue $ 476 $ 795 $ 1,549 $ 2,344 Total operating expenses 12,128 17,353 16,159 27,654 Loss from operations (11,652 ) (16,558 ) (14,610 ) (25,310 ) Net (loss) income (8,442 ) (17,164 ) (14,694 ) 17,098 Net (loss) income attributable to common shareholders (8,439 ) (17,157 ) (14,680 ) 17,099 Net (loss) income per share attributable to common shareholders: Basic $ (1.53 ) $ (3.15 ) $ (2.77 ) $ 0.43 Diluted $ (1.53 ) $ (3.15 ) $ (2.77 ) $ 0.40 Weighted-average common shares outstanding used in net (loss) income per share attributable to common shareholders: Basic 5,528,079 5,448,855 5,292,348 32,987,335 Diluted 5,528,079 5,448,855 5,292,348 34,989,218 (1) During the fourth quarter the Company recorded an immaterial correction of an error of $1.2 million for rent expense related to the three months ended September 30, 2016. The Company determined that these errors are not material to the respective interim financial statements.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	16. Related Party Transactions The Company is a party to intellectual property license agreements with Dr. Charpentier. As of December 31, 2017, and December 31, 2016, the Company owed Dr. Charpentier approximately $0 and $0.5 million, respectively, of sublicense fees primarily related to the Bayer Agreement. During the year ended December 31, 2017, the Company did not record any sublicensing fees due to Dr. Charpentier in research and development expense related to the Bayer Agreement. During 2016, the Company recorded sublicensing fees of $1.0 million due to Dr. Charpentier in research and development expense related to the Bayer Agreement. During the year ended December 31, 2017 and 2016, the Company recognized revenue of $4.8 million, and $1.1 million, respectively, related to the collaboration with Casebia. During the year ended December 31, 2017 and 2016, the Company recognized research and development expense of $4.5 million and $1.7 million, respectively, related to the performance of services for Casebia. The Company and have engaged several research institutions and companies to identify new delivery strategies and applications of the CRISPR/Cas9 technology. Additionally, the Company and are also a party to a number of research license agreements. The Company and will share costs associated with the research and license agreements. The Company received reimbursements of $4.4 million for both research and license agreements during 2017 which was recorded as a reduction of R&D expense in the income statement. There were no reimbursements recorded during 2016

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	17. Subsequent Events In January 2018, the Company completed an offering of 5,750,000 shares of our common shares, which were sold at a price of $22.75 per share. This offering resulted in $122.6 million of net proceeds to the Company. The underwriting discount of $7.8 million and other expenses of $0.4 million related to the equity offering were recorded as an offset to additional paid-in capital.

Summary of Significant Accoun26

Summary of Significant Accounting Policies and Basis of Presentation (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation and Use of Estimates	Basis of Presentation and Use of Estimates The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”), and include the accounts of (i) the Company, (ii) its wholly-owned subsidiaries, CRISPR Therapeutics Ltd., CRISPR Therapeutics Inc., and TRACR Hematology Inc., as of December 31, 2017. All intercompany accounts and transactions have been eliminated. 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 (“ASUs”) of the Financial Accounting Standards Board (“FASB”). Investments in partnerships where the Company has significant influence because it has a voting interest of 20% to 50%, are accounted for under the equity method. Results of associated companies are presented on a one-line basis. The Company accounts for its 50% investment share of Casebia Therapeutics LLP (“Casebia”) under the equity method of accounting. See Note 9 for further details. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, equity-based compensation expense, revenue recognition, equity method investments, and reported amounts of expenses during the reported period. Significant estimates in these consolidated financial statements have been made in connection with the calculation of revenues, research and development expenses, valuation of equity method of investment, equity-based compensation expense, fair value of Common Shares, fair value of intangible assets, and the provision for or benefit from income taxes. The Company bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions.
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, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company and the Company’s chief operating decision maker, namely, the chief executive officer, view the Company’s operations and manage its business in one operating segment, which is the business of discovering, developing and commercializing therapies derived from or incorporating genome-editing technology.
Foreign Currency Translation and Transactions	Foreign Currency Translation and Transactions The Company’s reporting currency is the U.S. Dollar. The Company‘s consolidated entities have the U.S. dollar as their functional currency with the exception of CRISPR Ltd. which has the British Pound Sterling (“GBP”) as its functional currency. CRISPR Ltd. has assets and liabilities translated into U.S. dollars at exchange rates in effect at the end of the year. Revenue and expenses are translated using the average exchange rates for the period. Net unrealized gains and losses resulting from foreign currency translation are included in accumulated other comprehensive income (loss), which is a separate component of shareholders’ (deficit) equity. Net foreign currency exchange transaction gains and losses resulting from the remeasurement of transactions denominated in currencies other than functional currency are included in other (expense) income, net in the consolidated statements of operations and comprehensive loss.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of 90 days or less from the purchase date to be cash equivalents. As of December 31, 2017 and 2016, the Company had $239.8 million and $315.5 million in cash equivalents, respectively. All cash was held in depository accounts and is reported at fair value.
Accounts Receivable	Accounts Receivable Accounts receivable of $2.6 million at December 31, 2017 consist of receivables from Vertex Pharmaceuticals, Incorporated (“Vertex”) and Casebia. As of December 31, 2016, the Company had accounts receivable of $3.2 million consisting of receivables from Vertex. Accounts receivables are recorded at invoiced amounts due under both the Vertex and Casebia collaboration agreements (see Note 9). Vertex and Casebia are creditworthy entities that maintain an ongoing relationship with the Company, as such the Company did not have an allowance for estimated losses recorded related to these receivables.
Concentrations of Credit Risk and Off-balance Sheet Risk	Concentrations of Credit Risk and Off-balance Sheet Risk Financial instruments that potentially subject the Company to concentrations of credit risk are primarily cash. The Company’s cash is held in accounts with financial institutions that management believes are creditworthy. 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. The Company has no financial instruments with off-balance sheet risk of loss.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist of a convertible debt instrument, accounts payable, accrued expenses and other non-current liabilities. The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurement and Disclosures The accounting standard describes a fair value hierarchy based on three levels of inputs, of which the first two are considered observable and the last unobservable, that may be used to measure fair value, which are the following: Level 1 — Quoted prices in active markets that are accessible at the market date for identical unrestricted assets or liabilities. Level 2 — Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs for which all significant inputs are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. To the extent that 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 amount of accounts receivable, accounts payable, and accrued expenses as reported on the consolidated balance sheets as of December 31, 2017 and 2016, approximate fair value, due to the short-term duration of these instruments. The fair value of the Company’s equity method investment in Casebia and convertible debt instruments were determined using level 3 inputs (See Note 9).
Property and Equipment	Property and Equipment Property and equipment 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. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation is recorded using the straight-line method over the estimated useful lives of the respective assets, which are as follows: Asset Estimated useful life Computer equipment 3 years Furniture, fixtures, and other 5 years Laboratory equipment 5 years Leasehold improvements Shorter of useful life or remaining lease term
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 value 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 value of the assets exceed their fair value. The Company has not recognized any impairment losses in the years ended December 31, 2017, 2016, and 2015.
Revenue Recognition	Revenue Recognition The Company recognizes revenue for each unit of accounting when all of the following criteria are met: (i) persuasive evidence of an arrangement exists, (ii) delivery has occurred or services have been rendered, (iii) the seller’s price to the buyer is fixed or determinable and (iv) collectability is reasonably assured. The terms of the Company’s collaboration and license agreements contain multiple deliverables, which include licenses to CRISPR/Cas9-based therapeutic products directed to specific targets, referred to as co-exclusive or exclusive licenses, joint steering committee participation, as well as research and development activities to be performed by the Company on behalf of the collaboration partner related to the licensed targets. Payments that the Company may receive under these agreements include nonrefundable technology access fees, payments for research activities, payments based upon the achievement of specified milestones and royalties on any resulting net product sales. The Company records the elements of its collaboration agreements that represent joint operating activities in accordance with FASB ASC Topic 808, Collaborative Arrangements (“ASC 808”). Accordingly, the elements of the collaboration agreements that represent activities in which both parties are active participants and to which both parties are exposed to the significant risks and rewards that are dependent on the commercial success of the activities, are recorded as collaborative arrangements. The Company considers the guidance in FASB ASC Topic 605-45, Revenue Recognition—Principal Agent Considerations (“ASC 605-45”) in determining the appropriate treatment for the transactions between the Company and its collaborative partner and the transactions between the Company and third parties. Generally, the classification of transactions under the collaborative arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. To date, the Company’s only source of revenue has been the collaboration and license and joint development and commercialization agreement with Vertex as well as research and development services provided to Casebia under the joint venture with Bayer HealthCare LLC (“Bayer”) (see Note 9). The Company evaluates multiple-element arrangements based on the guidance in FASB ASC Topic 605-25, Revenue Recognition—Multiple-Element Arrangements The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the selling price of a unit of accounting within each arrangement following the hierarchy of evidence prescribed by ASC 605-25. Accordingly, 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. The Company typically uses BESP to estimate the selling price as it generally does not have VSOE or TPE of selling price for its units of accounting. 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 periodically validates the BESP used 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 following criteria are met for that particular unit of accounting: 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. 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 contractual or estimated performance period for the undelivered items, 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 over 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. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company expects to complete its performance obligations under an arrangement. Steering committee services that are not inconsequential or perfunctory and that are determined to be performance obligations are combined with other research services or performance obligations required under an arrangement, if any, in determining the level of effort required in an arrangement and the period over which the Company expects to complete its aggregate performance obligations. 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: (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 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. 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. The Company will recognize revenue in its entirety upon successful accomplishment of any substantive milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, with a cumulative catch-up being recognized for the elapsed portion of the 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 Expenses	Research and Development Expenses Research and development costs, which include employee compensation costs, facilities, lab supplies and materials, overhead, preclinical development, and other related costs, are charged to expense as incurred. Research and development costs also include the costs the Company incurs in its performance of services or provision of materials in connection with the funded research undertaken as a part of the Company’s collaborative agreement with Vertex and Casebia. See Note 9 for further details.
Operating Leases	Operating Leases The Company leases office and laboratory facilities under a non-cancelable operating lease agreements. The lease agreements contain free or escalating rent payment provisions. The Company recognizes rent expense under such leases on a straight-line basis over the term of the lease with the difference between the expense and the payments recorded as deferred rent on the consolidated balance sheets. Lease renewal periods are considered on a lease-by-lease basis in determining the lease term. Funding of leasehold improvements by the Company’s landlord are accounted for as a tenant improvement allowance and are amortized as a reduction of rent expense over the term of the lease. Leasehold improvements are amortized straight-line over the shorter of the useful life or the remaining lease term.
Equity-Based Compensation Expense	Equity Based Compensation Expense The Company recognizes equity-based compensation expense for awards of equity instruments to employees and non-employee directors based on the grant date fair value of those awards in accordance with FASB ASC Topic 718, Stock Compensation The Company accounts for stock options issued to non-employees under FASB ASC Topic 505-50, Equity Based Payments to Non-Employees The Black-Scholes option pricing model requires the input of certain subjective assumptions, including (i) the expected share price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate and (iv) the expected dividend yield. Due to the lack of a public market for the trading of the Company’s Common Shares prior to its IPO and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The group of representative companies have characteristics similar to the Company, including stage of product development and focus on the life science industry. The Company uses the simplified method, which is the average of the final vesting tranche date and the contractual term, to calculate the expected term for options granted to employees as it 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 assumption. The risk-free interest rate is 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 has no current plans to pay any dividends on its Common Shares. The Company expenses the fair value of its equity-based compensation awards granted to employees on a straight-line basis over the associated service period, which is generally the period in which the related services are received. The Company measures equity-based compensation awards granted to non-employees at fair value as the awards vest and recognizes the resulting value as compensation expense at each financial reporting period. The Company records the expense for equity-based compensation awards subject to performance-based milestone vesting over the remaining service period using the accelerated method when management determines that achievement of the milestone is probable. Management evaluates when the achievement of a performance-based milestone is probable based on the expected satisfaction of the performance conditions as of the reporting date. The Company use a Monte Carlo simulation option-pricing model to determine the fair value of market-based awards. The model uses the same input assumptions as the Black-Scholes model, yet, it also incorporates the possibility that the market condition may not be satisfied. Compensation cost related to market-based awards are recognized using the accelerated method regardless of whether the market condition is satisfied, provided that the requisite service has been provided.
Patent Costs	Patent Costs Costs to secure and prosecute patent application and other legal costs related to the protection of the Company’s intellectual property are expensed as incurred and are classified as general and administrative expenses in the Company’s consolidated statements of operations.
Income Taxes	Income Taxes Income taxes are recorded in accordance with FASB ASC Topic 740, Income Taxes The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes 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. As of December 31, 2017 and 2016, the Company does not have any significant uncertain tax positions. The Company’s practice is to recognize interest and/or penalties related to income tax matters in income tax expense. See Note 14 for further details.
Comprehensive Loss	Comprehensive Loss Comprehensive loss consists of net income or loss and changes in equity during the period from transactions and other events and circumstances generated from non-owner sources. The Company’s net loss equals comprehensive loss, net of any changes in the foreign currency translation adjustment, for all periods presented. In addition, comprehensive loss attributable to the noncontrolling interest equals net loss for all periods presented.
Variable Interest Entities	Variable Interest Entities The Company reviews each legal entity formed by parties related to the Company to determine whether or not the Company has a variable interest in the entity and whether or not the entity would meet the definition of a VIE in accordance with FASB ASC Topic 810, Consolidation If the Company determines it is the primary beneficiary of a VIE that meets the definition of a business, the Company measures the assets, liabilities and noncontrolling interests of the newly consolidated entity at fair value in accordance with FASB ASC Topic 805, Business Combinations In February 2016, Casebia Therapeutics LLP, a limited liability partnership, was formed in the United Kingdom. In March 2016 upon consummation of the JV, Bayer and the Company each received a 50% equity interest in the entity in exchange for their contributions to the entity. The Company determined that Casebia was considered a VIE and concluded that it is not the primary beneficiary of the VIE. As such, the Company did not consolidate Casebia’s results into the consolidated financial statements. See Note 4 for further details.
Intangible Assets	Intangible Assets The Company’s intangible assets consist of acquired intellectual property rights and relate to the Company’s interest in TRACR. Intangible assets are recorded at fair value at the date of the business combination and are stated in the consolidated balance sheets net of accumulated amortization and impairments, if applicable. The Company evaluates the remaining useful life of intangible assets subject to amortization on a periodic basis to determine whether events and circumstances would indicate impairment or warrant a revision to the remaining useful life. If the estimate of an intangible asset’s remaining useful life is changed, the Company amortizes the remaining carrying value of the intangible asset prospectively over the revised remaining useful life. Intangible assets related to the acquired intellectual property rights are amortized over their estimated useful lives using the straight-line method as the pattern of revenues cannot be reasonably estimated. Amortization related to the acquired intellectual property rights is recorded in general and administrative expense in the consolidated statements of operations and comprehensive loss.
Net Loss Per Share Attributable to Common Shareholders	Net Loss Per Share Attributable to Common Shareholders Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common shareholders by the weighted-average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common shareholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options and warrants using the treasury stock method. The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to do so would be anti-dilutive (in common stock equivalent shares): As of December 31, 2017 2016 2015 Convertible preferred shares — — 18,837,024 Conversion of convertible loans — — 4,110,987 Dr. Emmanuelle Charpentier call option — — 328,017 Outstanding options 6,262,339 4,535,371 1,939,986 Unvested unissued restricted shares 157,515 89,367 142,794 Total 6,419,854 4,624,738 25,358,808
Subsequent Events	Subsequent Events The Company considered the events or transactions occurring after the balance sheet date, but prior to the issuance of the consolidated financial statements, for potential recognition or disclosure in its consolidated financial statements. All significant subsequent events have been properly disclosed in the consolidated financial statements.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”). Subsequently, the FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606), which adjusted the effective date of ASU 2014-09; ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which amends the principal-versus-agent implementation guidance and illustrations in ASU 2014-09; ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies identifying performance obligation and licensing implementation guidance and illustrations in ASU 2014-09; and ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which addresses implementation issues and is intended to reduce the cost and complexity of applying the new revenue standard in ASU 2014-09 (collectively, the “Revenue ASUs”). The Revenue ASUs noted above provide an accounting standard for a single comprehensive model for use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance. The accounting standard is effective for interim and annual periods beginning after December 15, 2017, with an option to early adopt for interim and annual periods beginning after December 15, 2016. The guidance permits two methods of adoption: retrospectively to each prior reporting period presented (the full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method). The Company currently anticipates adopting the new standard effective January 1, 2018 under the modified retrospective method. The Company has been monitoring FASB activity related to the new standard, as well as working with various non-authoritative groups to conclude on specific interpretative issues. The Company has established an implementation team to assess the impact of the standard on revenue recognition by reviewing our current accounting policies and practices to identify potential differences resulting from the application of the requirements of the new standard. The Company has identified and are in the process of implementing appropriate changes to our business controls, processes, and systems to support recognition and disclosure under the Revenue ASUs. In addition, the Company continues to monitor additional changes, modifications, clarifications or interpretations undertaken by the FASB, which may impact its conclusions. While the Company is substantially through our assessment of the new standard, the Company is still in the process of finalizing our implementation and determining the cumulative impact. The Company expects that under the new standard, the Company will continue to recognize revenue allocated to material rights to acquire certain licenses at a point in time when our collaboration partners exercise applicable options, which is consistent with our current revenue recognition model. The Company expects variable consideration from payments to R&D services will be allocable specifically to R&D services and will be recognized as earned under the new standard, whereas currently a portion of these payments are allocated to other undelivered elements, which could have a material impact on our financial statements. The impact will be recognized with a cumulative effect adjustment to equity at the date of initial application. The Company will adopt ASU 2014-09 effective January 1, 2018 using the modified retrospective transition method. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (Topic 718) (“ASU 2016-09”). The guidance changes how companies account for certain aspects of equity-based payments to employees. Entities will be required to recognize income tax effects of awards in the income statement when the awards vest or are settled. The guidance also allows an employer to repurchase more of an employee’s shares than it can under current guidance for tax withholding purposes providing for withholding at the employee’s maximum rate as opposed to the minimum rate without triggering liability accounting and to make a policy election to account for forfeitures as they occur. The Company adopted the new standard January 1, 2017. The Company made an accounting policy election to account for the impact of pre-vesting forfeitures as they occur rather than applying an estimated forfeiture rate, as previously required. Adoption did not materially impact the consolidated financial statements. In October 2016, the FASB issued ASU No. 2016-16, Income Taxes Intra-Entity Transfers of Assets Other Than Inventory In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In January 2017, the FASB issued ASU No. 2017-01, Business Combinations In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (“Topic 718”): Scope Modification Accounting. The new standard is intended to reduce the diversity in practice and cost and complexity when applying the guidance in Topic 718 to a change to the terms or conditions of a share-based payment award. The new standard will be effective beginning January 1, 2019.

Summary of Significant Accoun27

Summary of Significant Accounting Policies and Basis of Presentation (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of Estimated Useful Lives of Assets	Depreciation is recorded using the straight-line method over the estimated useful lives of the respective assets, which are as follows: Asset Estimated useful life Computer equipment 3 years Furniture, fixtures, and other 5 years Laboratory equipment 5 years Leasehold improvements Shorter of useful life or remaining lease term
Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share	The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to do so would be anti-dilutive (in common stock equivalent shares): As of December 31, 2017 2016 2015 Convertible preferred shares — — 18,837,024 Conversion of convertible loans — — 4,110,987 Dr. Emmanuelle Charpentier call option — — 328,017 Outstanding options 6,262,339 4,535,371 1,939,986 Unvested unissued restricted shares 157,515 89,367 142,794 Total 6,419,854 4,624,738 25,358,808

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net, consists of the following (in thousands): As of December 31, 2017 2016 Computer equipment $ 285 $ 110 Furniture, fixtures, and other 2,104 2,044 Laboratory equipment 6,603 2,970 Leasehold improvements 13,776 15,780 Construction work in process — 1,065 22,768 21,969 Accumulated Depreciation (3,911 ) (942 ) Property and equipment, net $ 18,857 $ 21,027

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets, Net of Accumulated Amortization	Intangible assets, net of accumulated amortization, are as follows (in thousands): Acquired intangible asset Cost Accumulated Amortization Net As of December 31, 2017 $ 547 $ (203 ) $ 344 As of December 31, 2016 $ 547 $ (148 ) $ 399
Schedule of Estimated Future Amortization of Acquired Intangible Assets	The estimated future amortization of acquired intangible assets as of December 31, 2017 is expected to be as follows (in thousands): For the Year Ended December 31, Amount 2018 $ 55 2019 55 2020 55 2021 55 2022 55 Thereafter 69 Total amortization $ 344

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Liabilities	Accrued expenses consist of the following (in thousands): As of December 31, 2017 2016 Payroll and employee-related costs $ 5,550 $ 2,585 Research costs 2,285 996 Licensing fees 609 492 Professional fees 2,176 2,715 Intellectual property costs 500 3,372 Accrued property and equipment — 5,081 Other 241 1,079 Total $ 11,361 $ 16,320

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Payments Required under Leases	Future minimum payments required under the leases as of December 31, 2017, are as follows (in thousands): Years Ended December 31, Amount 2018 $ 4,908 2019 6,369 2020 6,823 2021 7,027 2022 5,875 Thereafter 24,494 Total minimum lease payments $ 55,496

Share Capital (Tables)

Share Capital (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Schedule of Conditional Capital Reserved for Future Issuance	The Company had the following conditional capital reserved for future issuance: As of December 31, Type of Share Capital Conditional Capital 2017 2016 Common Shares Unvested unissued restricted stock 166,667 166,667 Common Shares Outstanding stock options 6,262,339 4,535,371 Common Shares Reserved for future issuance under stock option plans (1) 4,657,700 5,290,643 Common Shares Shares available for bonds and similar debt instruments 4,919,700 4,919,700 Common Shares Shares available for employee purchase plans 413,226 413,226 Total 16,419,632 15,325,607

Equity-based Compensation (Tabl

Equity-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense	The Company uses the straight-line attribution method to recognize stock-based compensation expense for stock options and restricted stock awards. Stock options and restricted stock generally vests over four years with 25% vesting on the first anniversary, and the remaining vesting monthly thereafter. The following table presents stock-based compensation expense in the Company’s Consolidated Statements of Operations: Years Ended December 31, 2017 2016 2015 Research and development $ 8,800 $ 4,848 $ 1,924 General and administrative 10,073 5,844 1,760 Loss from equity method investment 1,763 152 — Total $ 20,636 $ 10,844 $ 3,684
Fair Value of Employee and Non-employee Stock Option Award on the Grant Date Using the Black-Scholes Option-Pricing Model	The Company estimated the fair value of each employee and non-employee stock option award on the grant date using the Black-Scholes option-pricing model based on the following assumptions: Years Ended December 31, 2017 2016 2015 Employees: Options granted 2,894,850 2,411,240 1,913,319 Weighted - average exercise price $ 16.92 $ 12.19 $ 2.32 Weighted-average grant date fair value $ 10.86 $ 8.47 $ 3.11 Assumptions: Weighted-average expected volatility 72.1 % 81.0 % 76.4 % Expected term (in years) 6.0 6.0 6.0 Weighted-average risk free interest rate 2.0 % 1.4 % 1.7 % Expected dividend yield 0.0 % 0.0 % 0.0 % Non employees: Options granted 104,997 215,710 26,667 Weighted- average exercise price $ 18.74 $ 19.54 $ 1.85 Weighted- average grant date fair value $ 19.35 $ 17.38 $ 5.05 Assumptions: Weighted average expected volatility 81.5 % 88.2 % 84.1 % Expected term (in years) 9.4 10.0 10.0 Weighted-average risk free interest rate 2.4 % 2.4 % 2.2 % Expected dividend yield 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity for Employees and Non-employees	The following table summarizes stock option activity for employees and non-employees during the year ended December 31, 2017 (intrinsic value in thousands): Stock Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (years) Aggregate Intrinsic Value Outstanding at December 31, 2016 4,535,371 $ 8.38 9.1 $ 53,975 Granted 2,999,847 $ 16.98 Exercised (862,227 ) $ 3.53 Cancelled or forfeited (410,652 ) $ 7.06 Outstanding at December 31, 2017 6,262,339 $ 13.24 8.8 64,120 Exercisable at December 31, 2017 1,617,647 $ 9.64 8.3 $ 22,391 Vested or expected to vest at December 31, 2017 (1) 6,262,339 $ 13.24 8.8 $ 64,120 (1) This represents the number of vested stock options as of December 31, 2017 plus the unvested outstanding options at December 31, 2017 expected to vest in the future.
Summary of the Company's Restricted Stock Activity	The following table summarizes restricted stock activity for employees and non-employees during the year ended December 31, 2017: Reflected as outstanding upon vesting Reflected as outstanding upon grant date Total Weighted- Average Grant Date Fair Value Unvested restricted common shares at December 31, 2016 89,367 650,856 740,223 $ 3.84 Granted 101,667 — 101,667 $ 17.45 Vested (33,519 ) (418,032 ) (451,551 ) $ 6.40 Cancelled or forfeited — (23,938 ) (23,938 ) $ 1.72 Unvested restricted common shares at December 31, 2017 157,515 208,886 366,401 $ 8.49

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Loss Before Provision for Income Taxes	For the years ended December 31, 2017, 2016 and 2015, the loss before provision for income taxes consist of the following (in thousands): Years Ended December 31, 2017 2016 2015 Domestic $ 5,184 $ 3,322 $ 593 Foreign (71,792 ) (26,040 ) (26,414 ) Total $ (66,608 ) $ (22,718 ) $ (25,821 )
Schedule of (Provision for) Benefit from Income Taxes	The (provision for) benefit from income taxes consist of the following (in thousands): Years Ended December 31, 2017 2016 2015 Current income taxes: Federal $ (1,533 ) $ (649 ) $ (23 ) State (42 ) 11 (12 ) Foreign 6 17 (26 ) Total current income taxes (1,569 ) (621 ) (61 ) Deferred income taxes: Federal (477 ) 30 (37 ) State 297 105 65 Foreign — 2 26 Total deferred income taxes (180 ) 137 54 Total income tax provision $ (1,749 ) $ (484 ) $ (7 )
Schedule of Reconciliation of Income Tax Expense Computed at Statutory Corporate Income Tax Rate to Effective Income Tax Rate	A reconciliation of income tax expense computed at the statutory corporate income tax rate to the effective income tax rate for the years ended December 31, 2017, 2016 and 2015 is as follows: Years Ended December 31, 2017 2016 2015 Income tax expense at statutory rate 9.3 % 10.3 % 10.3 % State income tax, net of federal benefit 0.3 % 1.3 % 0.1 % Nondeductible expenses 0.0 % 1.6 % 0.0 % Foreign rate differential (2.5 %) (3.3 %) (1.4 %) Statutory to US GAAP permanent differences 1.8 % 6.6 % 0.0 % Stock-based compensation (2.9 %) (4.9 %) (1.4 %) Research credits 0.8 % 3.1 % 0.6 % Change in valuation allowance (9.4 %) (16.8 %) (8.2 %) Effective income tax rate (2.6 %) (2.1 %) —
Schedule of Significant Components of the Company's Deferred Tax Assets	Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets are comprised of the following (in thousands): Years Ended December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 9,987 $ 3,934 Accruals and reserves 1,123 791 Deferred Rent 3,494 5,228 Other deferred tax assets 36 7 Deferred revenue 1,721 2,525 Research credit 543 425 Total deferred tax assets 16,904 12,910 Less valuation allowance (13,041 ) (6,770 ) Net deferred tax assets 3,863 6,140 Deferred tax liabilities: Depreciation (3,791 ) (5,909 ) Intangible assets (59 ) (68 ) Other deferred tax liabilities (31 ) — Total deferred tax liabilities (3,881 ) (5,977 ) Long term deferred taxes $ (18 ) $ 163
Schedule of Aggregate Changes in Gross Unrecognized Tax Benefits	The aggregate changes in gross unrecognized tax benefits was as follows (in thousands): Years Ended December 31, 2017 2016 2015 Balance at beginning of year $ 163 $ 49 $ — Increases for tax positions taken during current period 178 134 49 Increases for tax positions taken in prior periods 13 — — Decreases for tax positions taken during current period — — — Decreases for tax positions taken in prior periods — (20 ) — Balance at end of year $ 354 $ 163 $ 49

Selected Quarterly Financial 35

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	Prior to its IPO on October 18, 2016, the Company had outstanding participating Preferred Shares. During the fourth quarter of the year ended December 31, 2016, the Company had net income, although for the full year the Company had a net loss. Accordingly, the Company used the two-class method to calculate net income per share for the fourth quarter of 2016. For purposes of calculating basic net income per share for the fourth quarter of 2016, the Company excluded from the numerator $3.1 million of net income attributable to participating securities. The Company calculated diluted net income per share under both the if-converted method and the two-class method and concluded that the two-class method was more dilutive than the if-converted method. Accordingly, the two-class income allocations were reapplied after taking into account the dilutive effect of non-participating securities. This resulted in net income of $3.1 million being allocated to the participating securities and excluded from the numerator of the Common Stock dilutive net income per share calculation. 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Collaboration revenue $ 2,703 $ 3,582 $ 2,387 $ 32,325 Total operating expenses 23,447 24,888 25,957 $ 31,353 (Loss) income from operations (20,744 ) (21,306 ) (23,570 ) $ 972 Net (loss) income (21,475 ) (22,315 ) (24,707 ) $ 140 Net (loss) income attributable to common shareholders (21,475 ) (22,315 ) (24,707 ) $ 140 Net (loss) income per share attributable to common shareholders: Basic $ (0.54 ) $ (0.56 ) $ (0.62 ) $ 0.00 Diluted $ (0.54 ) $ (0.56 ) $ (0.62 ) $ 0.00 Weighted-average common shares outstanding used in net (loss) income per share attributable to common shareholders: Basic 39,725,947 39,895,938 40,088,718 40,509,897 Diluted 39,725,947 39,895,938 40,088,718 41,635,843 2016 First Quarter Second Quarter Third Quarter Fourth Quarter (1) Collaboration revenue $ 476 $ 795 $ 1,549 $ 2,344 Total operating expenses 12,128 17,353 16,159 27,654 Loss from operations (11,652 ) (16,558 ) (14,610 ) (25,310 ) Net (loss) income (8,442 ) (17,164 ) (14,694 ) 17,098 Net (loss) income attributable to common shareholders (8,439 ) (17,157 ) (14,680 ) 17,099 Net (loss) income per share attributable to common shareholders: Basic $ (1.53 ) $ (3.15 ) $ (2.77 ) $ 0.43 Diluted $ (1.53 ) $ (3.15 ) $ (2.77 ) $ 0.40 Weighted-average common shares outstanding used in net (loss) income per share attributable to common shareholders: Basic 5,528,079 5,448,855 5,292,348 32,987,335 Diluted 5,528,079 5,448,855 5,292,348 34,989,218 (1) During the fourth quarter the Company recorded an immaterial correction of an error of $1.2 million for rent expense related to the three months ended September 30, 2016. The Company determined that these errors are not material to the respective interim financial statements.

Organization and Operations - A

Organization and Operations - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization and operations [Line Items]
Company formation date		Oct. 28, 2013
Company formation country name		Basel, Switzerland
Company registered office country name		Zug, Switzerland
Accumulated deficit		$ (125,440)	$ (57,083)
Cash		$ 239,758	$ 315,520
Equity Offering [Member] | Subsequent Event [Member]
Organization and operations [Line Items]
Issuance of shares, net of issuance cost (in shares)	5,750,000
Common shares price per share	$ 22.75
Proceeds from issuance of common shares	$ 122,600

Summary of Significant Accoun37

Summary of Significant Accounting Policies and Basis of Presentation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Segment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Mar. 31, 2016	Feb. 12, 2016
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Cash equivalents	$ 239,800,000	$ 315,500,000
Accounts receivable	2,626,000	3,157,000
Impairment loss	$ 0	0	$ 0
Assumed dividend yield, percent	0.00%
Significant uncertain tax positions	$ 0	0	$ 0
Vertex and Casebia [Member] | Collaborative Arrangement [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	2,600,000
Allowance	$ 0
Vertex [Member] | Collaborative Arrangement [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable		$ 3,200,000
Casebia Therapeutics LLP [Member]
Summary Of Significant Accounting Policies [Line Items]
Equity method investment, ownership percentage	50.00%			50.00%	50.00%

Summary of Significant Accoun38

Summary of Significant Accounting Policies and Basis of Presentation - Schedule of Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2017
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	3 years
Furniture, Fixtures, and Other [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	5 years
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Leasehold improvements	Shorter of useful life or remaining lease term

Summary of Significant Accoun39

Summary of Significant Accounting Policies and Basis of Presentation - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	6,419,854	4,624,738	25,358,808
Convertible Preferred Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share			18,837,024
Conversion of Convertible Loans [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share			4,110,987
Dr. Emmanuelle Charpentier Call Option [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share			328,017
Outstanding Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	6,262,339	4,535,371	1,939,986
Unvested Unissued Restricted Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	157,515	89,367	142,794

Property and Equipment, net - S

Property and Equipment, net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 22,768	$ 21,969
Accumulated Depreciation	(3,911)	(942)
Property and equipment, net	18,857	21,027
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	285	110
Furniture, Fixtures, and Other [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	2,104	2,044
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	6,603	2,970
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 13,776	15,780
Construction Work in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross		$ 1,065

Property and Equipment, net - A

Property and Equipment, net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation expense	$ 3	$ 0.9	$ 0.1

Variable Interest Entities - Ad

Variable Interest Entities - Additional Information (Detail) - USD ($) $ in Millions	Mar. 24, 2015	Oct. 31, 2016	Dec. 31, 2017	Mar. 31, 2016	Feb. 12, 2016
Casebia Therapeutics LLP [Member]
Variable Interest Entity [Line Items]
Date of formation of joint venture entity			Feb. 12, 2016
Equity method investment, ownership percentage			50.00%	50.00%	50.00%
Bayer Healthcare LLC [Member] | Casebia Therapeutics LLP [Member]
Variable Interest Entity [Line Items]
Equity method investment, ownership percentage				50.00%	50.00%
TRACR Hematology Limited [Member]
Variable Interest Entity [Line Items]
Number of shares acquired	4,600
Percentage of shares acquired	82.10%
TRACR Hematology Limited [Member] | Call Option [Member]
Variable Interest Entity [Line Items]
Shares held by subsidiary	1,000
Remaining ownership percentage in VIE	17.90%
TRACR Hematology Limited [Member] | Dr. Emmanuelle Charpentier [Member]
Variable Interest Entity [Line Items]
Conversion of ordinary shares into common shares	328,017
TRACR Hematology Limited [Member] | Dr. Emmanuelle Charpentier [Member] | Call Option [Member]
Variable Interest Entity [Line Items]
Reduction in noncontrolling interest		$ (0.1)
Reduction in stock based compensation		(0.2)
Reduction in additional paid-in capital		$ (0.1)

Intangible Assets - Additional

Intangible Assets - Additional Information (Detail) - Intellectual Property Rights [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite Lived Intangible Assets [Line Items]
Estimated useful life	10 years
Amortization expenses	$ 100,000	$ 100,000	$ 100,000
Remaining amortization period	6 years 3 months 18 days	7 years 3 months 18 days
Impairment charges	$ 0	$ 0	$ 0

Intangible Assets - Schedule of

Intangible Assets - Schedule of Intangible Assets, Net of Accumulated Amortization (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite Lived Intangible Assets [Line Items]
Acquired intangible asset, Net	$ 344	$ 399
Intellectual Property Rights [Member]
Finite Lived Intangible Assets [Line Items]
Acquired intangible asset, Cost	547	547
Acquired intangible asset, Accumulated Amortization	(203)	(148)
Acquired intangible asset, Net	$ 344	$ 399

Intangible Assets - Schedule 45

Intangible Assets - Schedule of Estimated Future Amortization of Acquired Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite Lived Intangible Assets [Line Items]
Acquired intangible asset, Net	$ 344	$ 399
Intellectual Property Rights [Member]
Finite Lived Intangible Assets [Line Items]
2,018	55
2,019	55
2,020	55
2,021	55
2,022	55
Thereafter	69
Acquired intangible asset, Net	$ 344	$ 399

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities Current [Abstract]
Payroll and employee-related costs	$ 5,550	$ 2,585
Research costs	2,285	996
Licensing fees	609	492
Professional fees	2,176	2,715
Intellectual property costs	500	3,372
Accrued property and equipment		5,081
Other	241	1,079
Total	$ 11,361	$ 16,320

Convertible Loans - Additional

Convertible Loans - Additional Information (Detail) - USD ($)	Jan. 29, 2016	Oct. 26, 2015	Jan. 31, 2016	Dec. 31, 2015	Dec. 07, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 26, 2016
Debt Conversion [Line Items]
Aggregate net proceeds borrowed							$ 35,010,000	$ 38,239,000
Conversion of convertible loans to Series B Preferred Shares							61,929,000
Proceeds raised from investors		$ 5,000,000
Gain on extinguishment of debt							$ 11,482,000
Common Shares [Member]
Debt Conversion [Line Items]
Preferred shares, conversion price per share		$ 9.33
IPO [Member]
Debt Conversion [Line Items]
Proceeds of public offering		$ 50,000,000
Minimum [Member]
Debt Conversion [Line Items]
Aggregate net proceeds borrowed		50,000,000
Vertex Convertible Loans [Member]
Debt Conversion [Line Items]
Borrowings under loan agreement		$ 40,000,000
Loan, accrued interest rate		2.50%				7.50%
Loan agreement maturity date		Apr. 26, 2016
Aggregate net proceeds borrowed					$ 38,200,000
Conversion of convertible loans to Series B Preferred Shares	$ 200,000
Vertex Convertible Loans [Member] | Base Interest Rate [Member]
Debt Conversion [Line Items]
Loan, accrued interest rate		2.50%
Bayer Convertible Loans [Member]
Debt Conversion [Line Items]
Loan agreement maturity date				Jan. 29, 2016
Aggregate net proceeds borrowed	$ 35,000,000
Borrowings under loan agreement						$ 35,000,000
Loan, accrued interest rate						2.00%
Fair value of convertible loan						$ 24,500,000
Series B Redeemable Convertible Preferred Shares [Member]
Debt Conversion [Line Items]
Preferred shares, shares issued upon conversion	2,859,278
Preferred shares, conversion price per share		$ 9.33	$ 13.43						$ 9.33
Series B Redeemable Convertible Preferred Shares [Member] | IPO [Member]
Debt Conversion [Line Items]
Preferred shares, conversion price per share		$ 9.33
Series B Redeemable Convertible Preferred Shares [Member] | Vertex Convertible Loans [Member]
Debt Conversion [Line Items]
Conversion of convertible loans to Series B Preferred Shares			$ 38,400,000
Preferred shares, shares issued upon conversion	2,859,278
Preferred shares, conversion price per share	$ 13.43
Fair value of convertible loan	$ 26,900,000
Gain on extinguishment of debt	11,500,000
Series B Redeemable Convertible Preferred Shares [Member] | Bayer Convertible Loans [Member]
Debt Conversion [Line Items]
Loan agreement maturity date				Jan. 29, 2016
Conversion of convertible loans to Series B Preferred Shares	$ 35,000,000		$ 35,000,000
Preferred shares, shares issued upon conversion	2,605,330
Preferred shares, conversion price per share	$ 13.43
Fair value of convertible loan	$ 24,500,000
Allocated to convertible loan	27,000,000
Interest expense	$ 8,000,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) SFr in Thousands		1 Months Ended			3 Months Ended	12 Months Ended
		Mar. 31, 2017CHF (SFr)	Sep. 30, 2016USD ($)	May 31, 2016	Dec. 31, 2016USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Contingencies And Commitments [Line Items]
Rental expense								$ 6,900,000	$ 4,200,000	$ 1,300,000
Research and development expense	[1]							69,800,000	42,238,000	$ 12,573,000
Research Agreements
Contingencies And Commitments [Line Items]
Upfront fees paid for license								4,400,000
Milestone payments								1,400,000
Formal Settlement Payment to US Common Shareholders [Member]
Contingencies And Commitments [Line Items]
Aggregate settlement amount					$ 2,000,000
Formal Settlement Payment to US Common Shareholders [Member] | Maximum [Member]
Contingencies And Commitments [Line Items]
Aggregate settlement amount			$ 2,000,000
Letter of Credit [Member]
Contingencies And Commitments [Line Items]
Letters of credit secured by cash held in restricted depository account					$ 3,200,000			$ 3,200,000	$ 3,200,000
Scenario Forecast [Member]
Contingencies And Commitments [Line Items]
Payment receivable under sublease agreement						$ 300,000	$ 1,500,000
Scenario Forecast [Member] | Research Institutions [Member]
Contingencies And Commitments [Line Items]
Research and development expense						100,000	300,000
Scenario Forecast [Member] | Research Agreements
Contingencies And Commitments [Line Items]
Research and development expense						$ 400,000	$ 500,000
Operating Lease, Research Facility Space [Member]
Contingencies And Commitments [Line Items]
Lease expiration date								Feb. 28, 2022
Extended term of lease expiration								5 years
Operating Lease, Primary Office and Research Facility Space [Member]
Contingencies And Commitments [Line Items]
Lease expiration date								Dec. 31, 2026
610 Main Street Sublease [Member]
Contingencies And Commitments [Line Items]
Lease expiration date				Dec. 22, 2026
Extended term of lease expiration				5 years
Office Space Lease, Zug [Member]
Contingencies And Commitments [Line Items]
Lease expiration date		Sep. 30, 2017
Extended term of lease expiration		6 months
Security cash deposit | SFr		SFr 11
[1]	(2)  Including the following amounts of research and development from a related party, see Note 16:

Commitments and Contingencies49

Commitments and Contingencies - Summary of Future Minimum Payments Required under Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 4,908
2,019	6,369
2,020	6,823
2,021	7,027
2,022	5,875
Thereafter	24,494
Total minimum lease payments	$ 55,496

Significant Contracts - Additio

Significant Contracts - Additional Information (Detail) € in Millions, shares in Millions, SFr in Millions

Feb. 12, 2016USD ($)

Dec. 19, 2015shares

Oct. 26, 2015USD ($)Milestone

Mar. 31, 2016USD ($)

Dec. 31, 2015

Apr. 30, 2014USD ($)

Apr. 30, 2014CHF (SFr)

Mar. 31, 2016USD ($)

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Dec. 22, 2016USD ($)

Jan. 29, 2016USD ($)

Nov. 30, 2014USD ($)

Nov. 30, 2014EUR (€)

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Research and development expense

[1]

$ 69,800,000

$ 42,238,000

$ 12,573,000

Cost sharing expensed relating to patent maintenance, defense and prosecution, incurred

1,200,000

2,800,000

1,500,000

Cost sharing expensed relating to patent maintenance, defense and prosecution, accrued

$ 400,000

2,800,000

Research and development arrangement, description

The research collaboration will end on the earlier of the date on which Vertex has exercised six options to obtain exclusive/co-exclusive licenses with respect to a collaboration target, or the fourth anniversary of the effective date of the agreement. The research term may be extended as mutually agreed by the parties up to nine additional months to complete any research activities under the approved research plan that are incomplete on the fourth anniversary of the effective date.

Non-current deferred revenue

$ 56,928,000

77,646,000

Date of joint venture agreement

Dec. 19, 2015

Cash contribution

100,000

Fair value allocation of arrangement consideration paid

$ 91,200,000

Remaining license fee

15,000,000

Other (expense) income, net

(197,000)

78,512,000

16,000

Equity method investment

36,500,000

Unrealized equity method losses

(1,763,000)

(36,532,000)

Stock-based compensation expense

20,636,000

10,844,000

3,684,000

Research and Development Expenses [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Stock-based compensation expense

8,800,000

4,848,000

1,924,000

Licensing Agreements [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Fair value

71,400,000

License and Patent Holder Consent [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Fair value allocation of arrangement consideration paid

63,600,000

Future Research and Development Services [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Fair value allocation of arrangement consideration paid

600,000

Bayer Convertible Loans [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Convertible Loan

35,000,000

Fair value of convertible loan

24,500,000

Fair value allocation of arrangement consideration paid

27,000,000

Debt instrument discount

8,000,000

Debt instrument face amount

35,000,000

Loan agreement maturity date

Jan. 29, 2016

Bayer Convertible Loans [Member] | Series B Redeemable Convertible Preferred Shares [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Fair value of convertible loan

$ 24,500,000

Loan agreement maturity date

Jan. 29, 2016

Bayer Healthcare LLC [Member] | Research and Development Expenses [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Upfront fees paid for license

0

1,000,000

Bayer Global Investments B.V [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Value of shares to be purchased via option through private placement | shares

35

Casebia Therapeutics LLP [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Research and development expense

4,500,000

1,200,000

Nonrefundable upfront payment received

$ 20,000,000

Aggregate consideration receivable

112,600,000

Revenue from collaboration agreement

4,800,000

1,200,000

Non-current deferred revenue

$ 100,000

500,000

Date of formation of joint venture entity

Feb. 12, 2016

Equity method investment, ownership percentage

50.00%

50.00%

50.00%

50.00%

Cash contribution

$ 100,000

Remaining license fees to be received from joint venture entity

$ 15,000,000

Royalty payments due to company from joint venture entity

0

Other payments due to company from joint venture entity

$ 0

Convertible Loan

$ 35,000,000

Cash contribution

100,000

Technology fee

34,900,000

Equity interest in the Joint Venture

36,400,000

Estimated revenue related to research and development services

6,300,000

Unrecognized equity method losses in excess of Company's interest

21,200,000

4,000,000

Stock-based compensation expense

1,800,000

200,000

Operating expenses of joint venture

36,300,000

80,800,000

Net loss of joint venture

(36,200,000)

80,800,000

Casebia Therapeutics LLP [Member] | Fair Value of CRISPR License Acquired [Member] | Research and Development Expenses [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Net loss of joint venture

77,400,000

Casebia Therapeutics LLP [Member] | Bayer Healthcare LLC [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Research and development expense

$ 45,000,000

4,500,000

1,700,000

Revenue from collaboration agreement

4,800,000

1,100,000

Equity method investment, ownership percentage

50.00%

50.00%

50.00%

Research and development funding term

5 years

Maximum [Member] | Casebia Therapeutics LLP [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from collaboration agreement

$ 35,000,000

Maximum [Member] | Casebia Therapeutics LLP [Member] | Bayer Healthcare LLC [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Research and development expense

$ 300,000,000

Research and Development Services [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Fair value

6,300,000

Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Research and development expense

$ 9,900,000

7,000,000

Clinical milestone payment payable

$ 10,000,000

Nonrefundable upfront payment received

75,000,000

Potential milestone receivable

420,000,000

Number of days to terminate the collaboration agreement

90 days

Notice period in the event of material breach of collaboration agreement

90 days

Number of days to terminate the product

270 days

Best estimate selling price for remaining research and development services

4,000,000

Best estimated selling price for collaboration agreement

158,600,000

Deferred revenue

80,000,000

Best estimate selling price for research and development services

4,000,000

Estimated selling price for collaboration agreement

40,000,000

Aggregate consideration receivable

131,000,000

Revenue from collaboration agreement

$ 36,200,000

4,000,000

200,000

Non-current deferred revenue

56,800,000

77,100,000

Vertex Pharmaceuticals Inc [Member] | Unit One [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Best estimated selling price for single collaboration

55,600,000

Vertex Pharmaceuticals Inc [Member] | Unit Two [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Best estimated selling price for single collaboration

48,400,000

Vertex Pharmaceuticals Inc [Member] | Unit Three [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Best estimated selling price for single collaboration

27,300,000

Vertex Pharmaceuticals Inc [Member] | Unit Four [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Best estimated selling price for single collaboration

27,300,000

Patent Assignment Agreement [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Research and development expense

0

33,000

100,000

Clinical milestone payment payable

$ 400,000

€ 0.3

Joint Development Agreement [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Nonrefundable upfront payment received

7,000,000

Up-front payment received

$ 7,000,000

Agreement description

In connection with entering into the JDA, the Company received a $7.0 million up-front payment from Vertex and is eligible for a one-time low seven-digit milestone payment upon the dosing of the second patient in a clinical trial with the initial product candidate. The net profits and net losses, as applicable, incurred under the JDA will be shared equally between us and Vertex.

Beta-Globin [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Best estimated selling price for single collaboration

48,900,000

Non-Exclusive Research License and Exclusive License to Develop [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

98,200,000

Non-Exclusive Research License and Exclusive License to Develop [Member] | Vertex Pharmaceuticals Inc [Member] | Unit One [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

34,400,000

Non-Exclusive Research License and Exclusive License to Develop [Member] | Vertex Pharmaceuticals Inc [Member] | Unit Two [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

30,000,000

Non-Exclusive Research License and Exclusive License to Develop [Member] | Vertex Pharmaceuticals Inc [Member] | Unit Three [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

16,900,000

Non-Exclusive Research License and Exclusive License to Develop [Member] | Vertex Pharmaceuticals Inc [Member] | Unit Four [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

16,900,000

Non-Exclusive Research License and Co-Exclusive License to Develop [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

30,300,000

Revenue from collaboration agreement

$ 30,300,000

Non-Exclusive Research License and Co-Exclusive License to Develop [Member] | Vertex Pharmaceuticals Inc [Member] | Research and Development Services [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

2,500,000

Exclusive Option Agreement [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue recognition, milestone method, revenue recognized

10,000,000

Investigational New Drug Application ("IND") [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue recognition, milestone method, revenue recognized

10,000,000

Investigational New Drug Application ("IND") [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue recognition, milestone method, revenue recognized

$ 10,000,000

Clinical Development And Regulatory Milestone [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of developmental milestone events | Milestone

9

Clinical Development Milestones [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue recognition, milestone method, revenue recognized

$ 90,000,000

Regulatory Approval Milestone [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue recognition, milestone method, revenue recognized

235,000,000

Commercial Milestones [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue recognition, milestone method, revenue recognized

$ 75,000,000

Number of developmental milestone events | Milestone

2

Developmental Milestone Events [Member] | Vertex Pharmaceuticals Inc [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue recognition, milestone method, revenue recognized

$ 420,000,000

Commercial Milestone Event One [Member] | Vertex Pharmaceuticals Inc [Member] | Minimum [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

500,000,000

Commercial Milestone Event Two [Member] | Vertex Pharmaceuticals Inc [Member] | Minimum [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate consideration receivable

$ 1,000,000,000

CRISPR-Charpentier License Agreement [Member]

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Upfront fees paid for license

$ 100,000

SFr 0.1

Annual payments for consulting agreement

0

0

0

Research and development expense

$ 0

$ 500,000

$ 900,000

[1]

(2)  Including the following amounts of research and development from a related party, see Note 16:

Redeemable Convertible Prefer51

Redeemable Convertible Preferred Shares - Additional Information (Detail) SFr / shares in Units, $ / shares in Units, $ in Thousands, SFr in Millions	Oct. 31, 2016shares	May 27, 2016USD ($)	Jan. 29, 2016USD ($)$ / shares	Jun. 30, 2016USD ($)$ / sharesshares	Jan. 31, 2016USD ($)$ / sharesshares	May 31, 2015USD ($)$ / sharesshares	May 31, 2015CHF (SFr)SFr / sharesshares	Apr. 30, 2015USD ($)$ / sharesshares	Dec. 31, 2017shares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares	Oct. 19, 2016shares	Apr. 26, 2016$ / shares	Dec. 31, 2015SFr / shares	Oct. 26, 2015$ / shares	Dec. 31, 2014shares
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized									0	0
Redeemable convertible preferred stock, shares issued									0	0
Redeemable convertible preferred stock, shares outstanding									0	0
Redeemable convertible preferred stock, shares issued											18,837,024
Redeemable convertible preferred stock, shares outstanding											18,837,024
Conversion of Vertex and Bayer convertible loans and accrued interest | $										$ 61,929
Vertex Convertible Loans [Member]
Temporary Equity [Line Items]
Conversion of Vertex and Bayer convertible loans and accrued interest | $			$ 200
Series A-1 Redeemable Convertible Preferred Shares [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued											440,001
Redeemable convertible preferred stock, shares outstanding									0	0	440,001					440,001
Redeemable convertible preferred stock, par value | SFr / shares														SFr 0.03
Series A-2 Redeemable Convertible Preferred Shares [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued											3,120,001
Redeemable convertible preferred stock, shares outstanding									0	0	3,120,001					3,120,001
Redeemable convertible preferred stock, par value | SFr / shares														0.03
Proceeds from issuance of preferred shares | $											$ 5,293
Series A-3 Redeemable Convertible Preferred Shares [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued								10,758,006			10,758,006
Redeemable convertible preferred stock, shares outstanding									0	0	10,758,006
Redeemable convertible preferred stock, par value | SFr / shares														0.03
Temporary equity, price per share | $ / shares								$ 4.24
Temporary equity share issued price per share | $ / shares								$ 2.12
Proceeds from issuance of preferred shares | $		$ 22,800						$ 22,800		$ 22,850	$ 22,850
Temporary equity, subscription receivable | $											$ 22,800
Series B Redeemable Convertible Preferred Shares [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued											4,519,016
Redeemable convertible preferred stock, shares outstanding									0	0	4,519,016
Redeemable convertible preferred stock, par value | SFr / shares														SFr 0.03
Temporary equity, price per share | (per share)				$ 13.43		$ 6.74	SFr 6.20
Proceeds from issuance of preferred shares				$ 38,100		$ 30,500	SFr 28			$ 38,075	$ 30,478
Common shares issued and sold				2,834,252	5,464,608	4,519,016	4,519,016				4,519,016
Preferred shares, conversion price per share | $ / shares					$ 13.43								$ 9.33		$ 9.33
Series B Redeemable Convertible Preferred Shares [Member] | Vertex Convertible Loans [Member]
Temporary Equity [Line Items]
Conversion of Vertex and Bayer convertible loans and accrued interest | $					$ 38,400
Preferred shares, conversion price per share | $ / shares			$ 13.43
Series B Redeemable Convertible Preferred Shares [Member] | Bayer Convertible Loans [Member]
Temporary Equity [Line Items]
Conversion of Vertex and Bayer convertible loans and accrued interest | $			$ 35,000		$ 35,000
Preferred shares, conversion price per share | $ / shares			$ 13.43
IPO [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized												27,135,884
Redeemable convertible preferred stock, shares authorized									one-for-one
Common shares issued and sold	4,429,311
IPO [Member] | Series B Redeemable Convertible Preferred Shares [Member]
Temporary Equity [Line Items]
Preferred shares, conversion price per share | $ / shares															$ 9.33

Share Capital - Additional Info

Share Capital - Additional Information (Detail)	Oct. 31, 2016USD ($)shares	Jan. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)	Dec. 31, 2017SFr / sharesshares	Dec. 31, 2016SFr / sharesshares
Class of Stock [Line Items]
Common stock, shares authorized				41,092,969	40,253,674
Common shares, par value | SFr / shares				SFr 0.03	SFr 0.03
Unvested restricted stock award				366,401	740,223
Treasury stock, shares				444,873	0
Common shares sold	170,689
Repurchase of treasury shares, value | $	$ 0
Common Stock, Voting Rights			one vote for each Common Share
Dividends declared and paid | $			$ 0
IPO [Member]
Class of Stock [Line Items]
Issuance of shares, net of issuance cost (in shares)	4,429,311
Overallotment Option [Member]
Class of Stock [Line Items]
Issuance of shares, net of issuance cost (in shares)	429,311
Equity Offering [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Issuance of shares, net of issuance cost (in shares)		5,750,000
Common shares price per share | $ / shares		$ 22.75
Proceeds from issuance of common shares | $		$ 122,600,000
Bayer Global Investments B.V [Member] | Private Placement [Member]
Class of Stock [Line Items]
Issuance of shares, net of issuance cost (in shares)	2,500,000
Unvested Restricted Share Award [Member]
Class of Stock [Line Items]
Unvested restricted stock award				55,848

Share Capital - Schedule of Con

Share Capital - Schedule of Conditional Capital Reserved for Future Issuance (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Conditional capital reserved for future issuance	16,419,632	15,325,607
Employee Purchase Plans [Member]
Class of Stock [Line Items]
Conditional capital reserved for future issuance	413,226	413,226
Bonds and Similar Debt Instruments [Member]
Class of Stock [Line Items]
Conditional capital reserved for future issuance	4,919,700	4,919,700
Unvested Unissued Restricted Stock [Member]
Class of Stock [Line Items]
Conditional capital reserved for future issuance	166,667	166,667
Outstanding Stock Options [Member]
Class of Stock [Line Items]
Conditional capital reserved for future issuance	6,262,339	4,535,371
Stock Option Plan [Member]
Class of Stock [Line Items]
Conditional capital reserved for future issuance	4,657,700	5,290,643

Share Capital - Schedule of C54

Share Capital - Schedule of Conditional Capital Reserved for Future Issuance (Parenthetical) (Detail)	1 Months Ended
	May 31, 2017shares
Equity [Abstract]
Number of options approved	2,012,684

Equity-based Compensation - Opt

Equity-based Compensation - Option and Grant Plans - Additional Information (Detail) - shares	1 Months Ended	12 Months Ended
	May 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options approved	2,012,684
Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation, options, vesting period		4 years
Share-based Compensation, options, contractual life		10 years
Plans [Member] | IPO [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment award, number of shares granted		0
Founder Awards [Member] | Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, requisite service period			4 years

Equity-based Compensation - Equ

Equity-based Compensation - Equity-Based Compensation Expense - Additional Information (Detail) - Stock Options and Restricted Stock Awards [Member]	12 Months Ended
	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation, options, vesting period	4 years
Share-based Compensation, options, vesting terms	vests over four years with 25% vesting on the first anniversary, and the remaining vesting monthly thereafter
Vesting Percentage on First Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation, options, vesting for first year percentage	25.00%

Equity-based Compensation - Sch

Equity-based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-Based compensation expense	$ 20,636	$ 10,844	$ 3,684
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-Based compensation expense	8,800	4,848	1,924
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-Based compensation expense	10,073	5,844	$ 1,760
Loss from Equity Method Investment [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-Based compensation expense	$ 1,763	$ 152

Equity-based Compensation - Fai

Equity-based Compensation - Fair Value of Employee and Non-employee Stock Option Award on the Grant Date Using the Black-Scholes Option-Pricing Model (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%
Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted	2,894,850	2,411,240	1,913,319
Weighted - average exercise price	$ 16.92	$ 12.19	$ 2.32
Weighted-average grant date fair value	$ 10.86	$ 8.47	$ 3.11
Weighted-average expected volatility	72.10%	81.00%	76.40%
Expected term (in years)	6 years	6 years	6 years
Weighted-average risk free interest rate	2.00%	1.40%	1.70%
Expected dividend yield	0.00%	0.00%	0.00%
Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted	104,997	215,710	26,667
Weighted - average exercise price	$ 18.74	$ 19.54	$ 1.85
Weighted-average grant date fair value	$ 19.35	$ 17.38	$ 5.05
Weighted-average expected volatility	81.50%	88.20%	84.10%
Expected term (in years)	9 years 4 months 24 days	10 years	10 years
Weighted-average risk free interest rate	2.40%	2.40%	2.20%
Expected dividend yield	0.00%	0.00%	0.00%

Equity-based Compensation - Sum

Equity-based Compensation - Summary of Stock Option Activity for Employees and Non-employees (Detail) - Employees and Non-employees [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options, Outstanding, Beginning Balance	4,535,371
Stock Options, Granted	2,999,847
Stock Options, Exercised	(862,227)
Stock Options, Cancelled or forfeited	(410,652)
Stock Options, Outstanding, Ending Balance	6,262,339	4,535,371
Stock Options, Exercisable	1,617,647
Stock Options, Vested or expected to vest	6,262,339
Weighted-Average Exercise Price, Outstanding, Beginning Balance	$ 8.38
Weighted-Average Exercise Price, Granted	16.98
Weighted-Average Exercise Price, Exercised	3.53
Weighted-Average Exercise Price, Cancelled or forfeited	7.06
Weighted-Average Exercise Price, Outstanding, Ending Balance	13.24	$ 8.38
Weighted-Average Exercise Price, Exercisable	9.64
Weighted-Average Exercise Price, Vested or expected to vest	$ 13.24
Weighted-Average Remaining Contractual Term	8 years 9 months 18 days	9 years 1 month 6 days
Weighted-Average Remaining Contractual Term, Exercisable	8 years 3 months 19 days
Weighted-Average Remaining Contractual Term, Vested or expected to vest	8 years 9 months 18 days
Aggregate Intrinsic Value, Outstanding Balance	$ 64,120	$ 53,975
Aggregate Intrinsic Value, Exercisable	22,391
Aggregate Intrinsic Value, Vested or expected to vest	$ 64,120

Equity-based Compensation - Sha

Equity-based Compensation - Share Based Payment Activity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	$ 42,200
Equity-based compensation expense not yet recognized, weighted-average service period for recognition	3 years 4 months 24 days
Stock-based compensation expense	$ 20,636	$ 10,844	$ 3,684
Founders' [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	800	$ 2,600
Weighted-Average Exercise Price, Vested or expected to vest		$ 12.65
Fay Corp Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation, number of shares issued		290,400
Fay Corp Awards [Member] | Founders' [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Transfer of vested shares		268,093
Chairman of Board of Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,200
Performance-Based [Member] | Vesting Percentage on First Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options, Granted	60,000	123,333
Share-based compensation, options, vested	362,872
Share-based compensation, options, outstanding	0
Market-Based [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options, Granted	150,000
Share-based compensation, options, vested	0
Share-based compensation, requisite service period	4 years
Market-Based [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, options, eligible to receive	0
Market-Based [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, options, eligible to receive	150,000
Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense not yet recognized, weighted-average service period for recognition	1 year 3 months 18 days
Total fair value of restricted stock vested	$ 8,300	$ 9,900
Total unrecognized compensation expenses	$ 3,600

Equity-based Compensation - S61

Equity-based Compensation - Summary of the Company's Restricted Stock Activity (Detail)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common shares, Number of Shares, Beginning Balance	740,223
Unvested restricted common shares, Number of Shares, Granted	101,667
Unvested restricted common shares, Number of Shares, Vested	(451,551)
Unvested restricted common shares, Number of Shares, Cancelled or forfeited	(23,938)
Unvested restricted common shares, Number of Shares, Ending Balance	366,401
Restricted Stock Outstanding Upon Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common shares, Number of Shares, Beginning Balance	89,367
Unvested restricted common shares, Number of Shares, Granted	101,667
Unvested restricted common shares, Number of Shares, Vested	(33,519)
Unvested restricted common shares, Number of Shares, Ending Balance	157,515
Restricted Stock Outstanding Upon Grant Date [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common shares, Number of Shares, Beginning Balance	650,856
Unvested restricted common shares, Number of Shares, Vested	(418,032)
Unvested restricted common shares, Number of Shares, Cancelled or forfeited	(23,938)
Unvested restricted common shares, Number of Shares, Ending Balance	208,886
Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common shares, Weighted-Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 3.84
Unvested restricted common shares, Weighted-Average Grant Date Fair Value, Granted | $ / shares	17.45
Unvested restricted common shares, Weighted-Average Grant Date Fair Value, Vested | $ / shares	6.40
Unvested restricted common shares, Weighted-Average Grant Date Fair Value, Cancelled or forfeited | $ / shares	1.72
Unvested restricted common shares, Weighted-Average Grant Date Fair Value, Ending Balance | $ / shares	$ 8.49

401(k) Savings Plan - Additiona

401(k) Savings Plan - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Compensation And Retirement Disclosure [Abstract]
Company contributions toward the plan	$ 0.5

Income Taxes - Schedule of Loss

Income Taxes - Schedule of Loss Before Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Domestic	$ 5,184	$ 3,322	$ 593
Foreign	(71,792)	(26,040)	(26,414)
Total	$ (66,608)	$ (22,718)	$ (25,821)

Income Taxes - Schedule of (Pro

Income Taxes - Schedule of (Provision for) Benefit from Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income taxes:
Federal	$ (1,533)	$ (649)	$ (23)
State	(42)	11	(12)
Foreign	6	17	(26)
Total current income taxes	(1,569)	(621)	(61)
Deferred income taxes:
Federal	(477)	30	(37)
State	297	105	65
Foreign		2	26
Total deferred income taxes	(180)	137	54
Total income tax provision	$ (1,749)	$ (484)	$ (7)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax Expense Computed at Statutory Corporate Income Tax Rate to Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Effective Income Tax Rate
Income tax expense at statutory rate	9.30%	10.30%	10.30%
State income tax, net of federal benefit	0.30%	1.30%	0.10%
Nondeductible expenses	0.00%	1.60%	0.00%
Foreign rate differential	(2.50%)	(3.30%)	(1.40%)
Statutory to US GAAP permanent differences	1.80%	6.60%	0.00%
Stock-based compensation	(2.90%)	(4.90%)	(1.40%)
Research credits	0.80%	3.10%	0.60%
Change in valuation allowance	(9.40%)	(16.80%)	(8.20%)
Effective income tax rate	(2.60%)	(2.10%)

Income Taxes - Schedule of Sign

Income Taxes - Schedule of Significant Components of Company's Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 9,987	$ 3,934
Accruals and reserves	1,123	791
Deferred Rent	3,494	5,228
Other deferred tax assets	36	7
Deferred revenue	1,721	2,525
Research credit	543	425
Total deferred tax assets	16,904	12,910
Less valuation allowance	(13,041)	(6,770)
Net deferred tax assets	3,863	6,140
Deferred tax liabilities:
Depreciation	(3,791)	(5,909)
Intangible assets	(59)	(68)
Other deferred tax liabilities	(31)
Total deferred tax liabilities	(3,881)	(5,977)
Long term deferred taxes, liabilities	$ (18)
Long term deferred taxes, assets		$ 163

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Corporate tax rate		9.30%	10.30%	10.30%
Transition tax		$ 0
Decrease in deferred tax assets		200,000
Non-US Net operating loss carryforwards		$ 127,400,000
Operating loss carryforwards expiration period		2,020
Domestic state research and development credit carryforwards		$ 400,000
Research and development credit carryforwards expiration period		2,032
Unrecognized tax benefits, gross		$ 354,000	$ 163,000	$ 49,000
Unrecognized tax benefits would favorably impact the effective tax rate if recognized		300,000
Accrued interest or penalties related to uncertain tax positions		0	0	0
Uncertain tax positions recognized		0	$ 0	$ 0
U.S. Domestic Federal [Member] | Research and Development [Member]
Valuation Allowance [Line Items]
Tax credit carryforward amount		$ 300,000
Tax credit carryforward, expiration year		2,037
Uncertain tax positions		$ 200,000
Maximum [Member]
Valuation Allowance [Line Items]
Corporate tax rate		35.00%
Scenario Plan [Member]
Valuation Allowance [Line Items]
Corporate tax rate	21.00%
Switzerland [Member]
Valuation Allowance [Line Items]
Increase in valuation allowance		$ 6,300,000

Income Taxes - Schedule of Aggr

Income Taxes - Schedule of Aggregate Changes in Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 163	$ 49
Increases for tax positions taken during current period	178	134	$ 49
Increases for tax positions taken in prior periods	13
Decreases for tax positions taken in prior periods		(20)
Balance at end of year	$ 354	$ 163	$ 49

Selected Quarterly Financial 69

Selected Quarterly Financial Data (Unaudited) - Additional Information (Detail) $ in Millions	3 Months Ended
	Dec. 31, 2016USD ($)
Quarterly Financial Information Disclosure [Abstract]
Net Income attributable to Participating securities excluded from numerator for calculating basic net income per share	$ 3.1
Net Income attributable to participating securities excluded from numerator for calculating diluted net income per share	$ 3.1

Selected Quarterly Financial 70

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	[1]	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 32,325	$ 2,387	$ 3,582	$ 2,703	$ 2,344		$ 1,549	$ 795	$ 476	$ 40,997	[2]	$ 5,164	[2]	$ 247	[2]
Total operating expenses	31,353	25,957	24,888	23,447	27,654		16,159	17,353	12,128	105,645		73,294		25,976
(Loss) income from operations	972	(23,570)	(21,306)	(20,744)	(25,310)		(14,610)	(16,558)	(11,652)	(64,648)		(68,130)		(25,729)
Net loss	140	(24,707)	(22,315)	(21,475)	17,098		(14,694)	(17,164)	(8,442)	(68,357)		(23,202)		(25,828)
Net (loss) income attributable to common shareholders	$ 140	$ (24,707)	$ (22,315)	$ (21,475)	$ 17,099		$ (14,680)	$ (17,157)	$ (8,439)	$ (68,357)		$ (23,177)		$ (25,503)
Net (loss) income per share attributable to common shareholders:
Basic	$ 0	$ (0.62)	$ (0.56)	$ (0.54)	$ 0.43		$ (2.77)	$ (3.15)	$ (1.53)
Diluted	$ 0	$ (0.62)	$ (0.56)	$ (0.54)	$ 0.40		$ (2.77)	$ (3.15)	$ (1.53)
Weighted-average common shares outstanding used in net (loss) income per share attributable to common shareholders:
Basic	40,509,897	40,088,718	39,895,938	39,725,947	32,987,335		5,292,348	5,448,855	5,528,079
Diluted	41,635,843	40,088,718	39,895,938	39,725,947	34,989,218		5,292,348	5,448,855	5,528,079
[1]	During the fourth quarter the Company recorded an immaterial correction of an error of $1.2 million for rent expense related to the three months ended September 30, 2016. The Company determined that these errors are not material to the respective interim financial statements.
[2]	(1)  Including the following amounts of revenue from a related party, see Note 16:

Selected Quarterly Financial 71

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Parenthetical) (Detail) $ in Millions	3 Months Ended
	Dec. 31, 2016USD ($)
Rent Expense [Member]
Quarterly Financial Data [Line Items]
Immaterial correction of an error amount	$ 1.2

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Research and development expense	[1]		$ 69,800,000	$ 42,238,000	$ 12,573,000
Casebia Therapeutics LLP [Member]
Related Party Transaction [Line Items]
Revenue from collaboration agreement			4,800,000	1,200,000
Research and development expense			4,500,000	1,200,000
Bayer Healthcare LLC [Member]
Related Party Transaction [Line Items]
Sublicense fees payable			0	500,000
Bayer Healthcare LLC [Member] | Casebia Therapeutics LLP [Member]
Related Party Transaction [Line Items]
Revenue from collaboration agreement			4,800,000	1,100,000
Research and development expense		$ 45,000,000	4,500,000	1,700,000
Reimbursements from research and license agreements			4,400,000	0
Bayer Healthcare LLC [Member] | Research and Development [Member]
Related Party Transaction [Line Items]
Sublicense fees recorded			$ 0	$ 1,000,000
[1]	(2)  Including the following amounts of research and development from a related party, see Note 16:

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] - Equity Offering [Member] $ / shares in Units, $ in Millions	1 Months Ended
	Jan. 31, 2018USD ($)$ / sharesshares
Subsequent Event [Line Items]
Issuance of shares, net of issuance cost (in shares) | shares	5,750,000
Common shares price per share | $ / shares	$ 22.75
Proceeds from issuance of common shares	$ 122.6
Payments for underwriting discount	7.8
Payments for other expenses	$ 0.4