MyoKardia (MYOK) 10-Q 7 Nov 16 2016 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2016	Nov. 02, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q3
Trading Symbol	MYOK
Entity Registrant Name	MyoKardia Inc
Entity Central Index Key	1,552,451
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		31,428,998

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Current assets
Cash and cash equivalents	$ 77,120	$ 112,265
Prepaid expenses and other current assets	762	1,282
Total current assets	77,882	113,547
Property and equipment, net	2,729	2,744
Other long-term assets	750	289
Total assets	81,361	116,580
Current liabilities
Accounts payable	1,731	2,143
Accrued liabilities	6,930	5,633
Deferred revenue	3,550	14,199
Total current liabilities	12,211	21,975
Other long-term liabilities	503	732
Total liabilities	12,714	22,707
Commitments and contingencies (Note 6)
Stockholders’ equity (deficit)
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; none issued and outstanding
Common stock, $0.0001 par value, 150,000,000 shares authorized at September 30, 2016 and December 31, 2015; 27,031,114 and 27,053,156 shares, issued and outstanding at September 30, 2016 and December 31, 2015, respectively	3	3
Additional paid-in-capital	160,740	158,555
Accumulated deficit	(92,096)	(64,685)
Total stockholders’ equity	68,647	93,873
Total liabilities and stockholders’ equity	$ 81,361	$ 116,580

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	27,031,114	27,053,156
Common stock, shares outstanding	27,031,114	27,053,156

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Collaboration and license revenue	$ 3,550	$ 3,550	$ 10,649	$ 10,649
Operating expenses
Research and development	8,783	6,672	26,192	20,017
General and administrative	4,031	2,300	11,947	5,961
Total operating expenses	12,814	8,972	38,139	25,978
Loss from operations	(9,264)	(5,422)	(27,490)	(15,329)
Interest and other income, net	33	(40)	79	(22)
Change in fair value of redeemable convertible preferred stock call option liability				314
Net loss and comprehensive loss	(9,231)	(5,462)	(27,411)	(15,037)
Cumulative dividend relating to redeemable convertible preferred Stock		(1,900)		(4,530)
Accretion of redeemable convertible preferred stock to redemption value		(26)		(88)
Net loss attributable to common stockholders	$ (9,231)	$ (7,388)	$ (27,411)	$ (19,655)
Net loss per share attributable to common stockholders, basic and Diluted	$ (0.35)	$ (2.78)	$ (1.04)	$ (7.98)
Weighted-average number of common shares used to compute net loss per share, basic and diluted	26,470,298	2,654,365	26,331,852	2,463,876

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities:
Net loss	$ (27,411)	$ (15,037)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	815	741
Stock-based compensation expense	1,781	246
Gain on sale of equipment		(15)
Change in fair value of redeemable convertible preferred stock call option liability, net		(314)
Change in operating assets and liabilities:
Prepaid expenses and other assets	520	(192)
Other long-term assets		38
Accounts payable	(372)	413
Accrued liabilities	1,027	1,089
Other long-term liabilities	(75)	503
Deferred revenue	(10,649)	(10,649)
Net cash used in operating activities	(34,364)	(23,177)
Cash flows from investing activities:
Purchases of property and equipment	(854)	(1,374)
Proceeds from sale of equipment		134
Changes in restricted cash		60
Net cash used in investing activities	(854)	(1,180)
Cash flows from financing activities:
Proceeds from issuance of common stock	226	352
Payments of deferred offering costs	(153)	(935)
Net cash provided by financing activities	73	45,254
Net (decrease) increase in cash and cash equivalents	(35,145)	20,897
Cash and cash equivalents, beginning of period	112,265	43,648
Cash and cash equivalents, end of period	77,120	64,545
Supplemental disclosures of noncash investing and financing information
Unpaid deferred offering costs	461	778
Vesting of early exercised options and restricted stock	226	120
Unpaid portion of property and equipment purchases included in period-end accounts Payable	$ 7	48
Accretion of dividends on redeemable convertible preferred stock		4,530
Accretion of redeemable convertible preferred stock to redemption value		88
Series B Redeemable Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of preferred stock, net		$ 45,837

Formation and Business of the C

Formation and Business of the Company	9 Months Ended
	Sep. 30, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Formation and Business of the Company	1. Formation and Business of the Company MyoKardia, Inc. (the “Company”), incorporated under the laws of the State of Delaware in 2012, is a clinical stage biopharmaceutical company pioneering a precision medicine approach to discover, develop and commercialize targeted therapies for the treatment of serious and neglected rare cardiovascular diseases. Our initial focus is on the treatment of heritable cardiomyopathies, a group of rare, genetically-driven forms of heart failure that result from biomechanical defects in cardiac muscle contraction. We have used our precision medicine platform to generate a robust pipeline of therapeutic programs for the chronic treatment of the two most common forms of heritable cardiomyopathy—hypertrophic cardiomyopathy, or HCM, and dilated cardiomyopathy, or DCM. We have initiated clinical sites and started screening patients in a Phase 2 clinical trial of MYK-461, our lead product candidate for the treatment of HCM, and plan to advance MYK-491, our lead product candidate for the treatment of DCM, into Phase 1 clinical development by the first half of 2017. Using our precision medicine development strategy, we believe we have efficiently generated clinical proof of mechanism for MYK-461 in both healthy volunteers and in HCM patients, and we intend to pursue a similar path for MYK-491. In 2016, MYK-461 was granted Orphan Drug Designation by the U.S. Food and Drug Administration, or the FDA. The accompanying unaudited Condensed Consolidated Financial Statements have been prepared on a going-concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. Through September 30, 2016, the Company has financed its operations through an initial public offering (“IPO”) and private placements of redeemable convertible preferred stock and funds received in connection with a license and collaboration agreement with Aventis Inc., a wholly-owned subsidiary of Sanofi S.A., entered into in August 2014 (the “Collaboration Agreement”) (See Note 4). The Company received net proceeds of $93.9 million from the sale of shares of its Series A, A-1 and B redeemable convertible preferred stock. On November 3, 2015, the Company completed its IPO of 6,253,125 shares of common stock at an offering price of $10.00 per share, resulting in net proceeds of approximately $55.6 million, after deducting underwriting discounts, commissions and offering costs. On October 3, 2016, the Company completed a follow-on public offering of 4,370,000 shares of common stock at an offering price of $15.00 per share, resulting in net proceeds of approximately $61.0 million, after deducting underwriting discounts, commissions and estimated offering costs. The Company has incurred significant losses and negative cash flows from operations. As at September 30, 2016, the Company had an accumulated deficit of $92.1 million and cash and cash equivalents of $77.1 million, which it believes will be sufficient to fund its planned operations through at least the next twelve months. The accompanying unaudited Condensed Consolidated Financial Statements, in the opinion of management, include all adjustments which the Company considers necessary for the fair statement of the Condensed Consolidated Results of Operations and Comprehensive Loss and Cash Flows for the interim periods covered and the Condensed Consolidated Financial Position of the Company at the date of the balance sheets. The consolidated financial statements of the Company as at December 31, 2015 included the Company’s accounts and have been prepared in conformity with accounting principles generally accepted in the United States of America (“US GAAP”). The interim results presented herein are not necessarily indicative of the results of operations that may be expected for the full fiscal year ending December 31, 2016, or any other future period. The accompanying unaudited Condensed Consolidated Financial Statements and related financial information should be read in conjunction with the Company’s audited consolidated financial statements and the related notes thereto for the year ended December 31, 2015 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015 filed with the U.S. Securities and Exchange Commission (the “SEC”) on March 18, 2016 (the “Annual Report”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Significant accounting policies are described in Note 2 to the consolidated financial statements for the year ended December 31, 2015 included in the Annual Report. There have been no changes to the Company’s significant accounting policies during the nine months ended September 30, 2016. Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), the Company meets the definition of an emerging growth company, and has irrevocably elected to opt out of the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. In August 2016, the FASB issued Accounting Standard Update (“ASU”) No. 2016-15, Statement of Cash Flows (Topic 230) – Classification of Certain Cash Receipts and Cash Payments. In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts - Narrow-Scope Improvements and Practical Expedients. Revenue from Contracts with Customers In March 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts, Revenue Recognition. In March 2016, the FASB issued ASU No. 2016-09, Share-Based Compensation, In February 2016, the FASB issued ASU No. 2016-02 (Topic 842) Leases . ASU 2016-02 In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. In June 2014, the FASB issued ASU 2014-12, Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers Revenue Recognition.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Financial assets and liabilities are recorded at fair value. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows: Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Inputs (other than quoted market prices included in Level 1) are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument’s anticipated life. Level 3—Inputs reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. The following table sets forth a summary of the changes in the fair value of the Company’s Level 1 financial instruments as follows (in thousands): Fair Value Measurements at September 30, 2016 Total Level 1 Level 2 Level 3 Assets Money Market funds $ 76,934 $ 76,934 $ — $ — Fair Value Measurements at December 31, 2015 Total Level 1 Level 2 Level 3 Assets Money Market funds $ 111,533 $ 111,533 $ — $ — The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments as follows (in thousands): Convertible preferred stock call option liability, net Balance at December 31, 2014 $ 314 Change in fair value recorded in statement of operations and comprehensive loss (314 ) Balance at September 30, 2015 $ —

Collaboration and License Agree

Collaboration and License Agreement	9 Months Ended
	Sep. 30, 2016
Collaboration And License Agreement Disclosure [Abstract]
Collaboration and License Agreement	4. Collaboration and License Agreement Sanofi (Aventis Inc.) In August 2014, the Company entered into the Collaboration Agreement with Aventis Inc., a wholly-owned subsidiary of Sanofi S.A. (together with Aventis, Inc., “Sanofi”), for the research, development and potential commercialization of pharmaceutical products for the treatment, prevention and diagnosis of hypertrophic and dilated cardiomyopathy, as well as potential additional indications. Pursuant to the Collaboration Agreement, in addition to potential future royalty payments, Sanofi agreed to provide up to $200.0 million in financial consideration to the Company consisting of the following components: 1. a $35.0 million upfront cash payment; 2. a $10.0 million initial equity investment; 3. a $25.0 million milestone-based contingent payment; 4. up to an $85.0 million project continuation payment if Sanofi elects to extend the term of the research collaboration beyond December 31, 2016, as described below; and 5. up to $45.0 million in funding from Sanofi of approved in-kind research and clinical activities over a four-year period. The Company is also entitled to receive tiered royalties beginning in the mid-single digits to the mid-teens on net sales of certain hypertrophic cardiomyopathy (“HCM”) and dilated cardiomyopathy (“DCM”) finished products outside the United States and on net sales of certain DCM finished products in the United States. Sanofi is eligible to receive tiered royalties beginning in the mid-single digits to the low teens on the Company’s net sales of certain HCM finished products in the United States. The Collaboration Agreement covers three main research programs, “HCM1” (“HCM-1” or “MYK-461”), “HCM2” (“HCM-2”) and “DCM1” (“DCM-1” or “MYK-491”). The Company is solely responsible for conducting research and development activities through early human efficacy studies, except for specified research activities to be conducted by Sanofi. The estimated completion of proof-of-concept phases are staggered, depending on the program. Thereafter, the Company will lead worldwide development and United States commercial activities for the MYK-461 and HCM-2 programs, Sanofi will lead global development and commercial activities for DCM-1 and Sanofi will lead ex-United States development and commercial activities for the MYK-461 and HCM-2 programs where it has ex-United States commercialization rights. Sanofi also has the option to co-promote in the United States for potential expanded cardiovascular diseases outside of the genetically targeted indications for the MYK-461 and HCM-2 programs, with the Company having the option to co-promote the DCM-1 program in the United States. The Company accounted for the Collaboration Agreement by evaluating each of the financial components discussed above: 1. $35.0 million upfront payment. The Company received a non-refundable upfront payment and identified the following performance obligations at the inception of the Collaboration Agreement: (i) the transfer of intellectual property rights and know-how (license), (ii) the obligation to provide certain limited research and development services during the term of the license agreement and (iii) the obligation to participate on the development and commercialization committees. The Company applied the guidance under ASC 605-25, Multiple Element Arrangements, to account for this upfront payment. The Company evaluated the underlying goods and services delivered under the Collaboration Agreement and concluded that the performance obligations do not have standalone value, and accordingly accounted for the deliverables as one unit of accounting. The $35.0 million payment was recorded by the Company as deferred revenue on its consolidated balance sheet upon receipt, which the Company is recognizing as revenue on a straight-line basis over the expected term of research and development services through December 31, 2016 because there is not a more discernible pattern of performance in which the research and development services occur. During the three months ended September 30, 2016 and September 30, 2015, and the nine months ended September 30, 2016 and September 30, 2015, the Company recognized $3.6 million of revenue in both three-month periods and $10.6 million of revenue in both nine-month periods, respectively, under the Collaboration Agreement. As of September 30, 2016 and December 31, 2015, the Company had recorded deferred revenue on its consolidated balance sheet of $3.5 million and $14.2 million, respectively. 2. $10.0 million upfront investment in Series A-1 redeemable convertible preferred stock. In August 2014, the Company entered into a Series A-1 redeemable convertible preferred stock purchase agreement with Sanofi. The stock purchase agreement was signed as a separate transaction from the Collaboration Agreement. Pursuant to the stock purchase agreement, the Company sold 6,666,667 shares of Series A-1 redeemable convertible preferred stock to Sanofi at $1.50 per share. The Company concluded that the $1.50 per share price represented the fair value of the redeemable convertible preferred stock issued. 3. $25.0 million milestone-based payment. The Company is eligible to receive a one-time, non-refundable, non-creditable payment of $25.0 million upon the submission of an investigational new drug application for any DCM-1 development candidate to the FDA or a comparable regulatory authority in Europe or another major market country for any DCM-1 product. The Company will account for this milestone payment separately from the rest of the agreement and recognize revenue upon achievement of the milestone. The Company had not achieved this milestone as of September 30, 2016. 4. Up to $85.0 million continuation payments. Under the Collaboration Agreement, Sanofi must determine by December 31, 2016 whether or not to continue the Collaboration Agreement. Sanofi agreed that if it so elects to continue the Collaboration Agreement, it will pay: • a one-time, non-refundable, non-creditable cash payment of $45.0 million • an additional $40.0 million in connection with the purchase of the Company’s preferred stock, assuming (i) the Company had not previously closed either a Qualified IPO (at which time this obligation would terminate) or a private financing prior to a Qualified IPO and (ii) Sanofi had not previously purchased shares of the Company’s stock pursuant to such rights to purchase the Company’s capital stock in accordance with the terms of the Collaboration Agreement. The $40.0 million payment was reduced by $5.0 million to $35.0 million in connection with Sanofi’s subsequent purchase of shares of the Company’s Series B redeemable convertible preferred stock in April 2015, and the remaining obligation terminated in connection with the Company’s IPO in October 2015. Sanofi also had a time-restricted right to purchase $40.0 million in shares of the Company’s redeemable convertible preferred stock at the discounted price, which would have satisfied the $40.0 million obligation to purchase shares of the Company’s capital stock in connection with the continuation decision. Sanofi’s option to purchase $40.0 million of additional shares of the Company’s redeemable convertible preferred stock at the discounted price expired upon the closing of the Series B redeemable convertible preferred stock financing in April 2015. The Company believes that the continuation payments have significant uncertainty and are outside the control of the Company because Sanofi has sole discretion to determine whether or not to continue, and will therefore account for these potential payments separate from the other deliverables in this agreement. The Company determined that Sanofi’s right to purchase the redeemable convertible preferred stock at the discounted price, and the Company’s corresponding obligation to issue this additional redeemable convertible preferred stock, represented a freestanding financial instrument. The freestanding convertible preferred stock call option liability was initially recorded at its fair value of $0.7 million in 2014. The Company recorded a decrease in the fair value of this liability of $0.3 million and $0.4 million in the consolidated statements of operations and comprehensive loss during the years ended December 31, 2015 and 2014, respectively. As of September 30, 2016, Sanofi had not provided the confirmation of continuation under the Collaboration Agreement. 5. Up to $45.0 million in-kind research and collaboration activities. Sanofi can fund up to $45.0 million of pre-approved funding of research and collaboration activities. Since Sanofi will pay its vendors and personnel directly as per the Collaboration Agreement, the Company will not receive cash from Sanofi and therefore will not account for the funding of the in-kind services.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2016
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and Equipment Property and equipment consist of the following (in thousands): September 30, 2016 December 31, 2015 Scientific equipment $ 4,597 $ 3,995 Furniture and equipment 488 301 Capitalized software 231 225 Leasehold improvements 308 303 Total 5,624 4,824 Less: Accumulated depreciation and amortization (2,895 ) (2,080 ) Property and equipment, net $ 2,729 $ 2,744 Depreciation and amortization expense was $0.3 million and $0.8 million, and $0.2 million and $0.7 million, for the three and nine months ended September 30, 2016 and 2015, respectively. Deferred Offering Costs Deferred offering costs, consisting of legal, accounting and other fees and costs related to the Company’s follow-on public offering are capitalized and will be offset against the proceeds received by the Company in October 2016. There were $0.5 million and zero deferred offering costs capitalized as of September 30, 2016 and December 31, 2015, respectively, in other long term assets on the consolidated balance sheets. Accrued Liabilities Accrued liabilities consist of the following (in thousands): September 30, 2016 December 31, 2015 Payroll and related expenses $ 3,257 $ 2,515 Clinical research and development 2,618 2,145 Legal and accounting fees 409 33 Other 646 940 Total accrued liabilities $ 6,930 $ 5,633

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2016
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Purchase Commitments The Company conducts product research and development programs through a combination of internal and collaborative programs that include, among others, arrangements with universities, contract research organizations and clinical research sites. The Company has contractual arrangements with these organizations; however, these contracts are generally cancelable on 30 days’ notice and the obligations under these contracts are largely based on services performed. Facility Leases On June 29, 2012, the Company entered into a 66-month lease for approximately 12,000 square feet of office and laboratory space in South San Francisco with annual payments of approximately $0.5 million. In connection with this lease agreement, the Company also entered into a shared facilities and services agreement with Global Blood Therapeutics, Inc. (“GBT”), a co-tenant in the office building (See Note 10). In October 2014, the Company entered into a lease assignment agreement with the owner of the building and GBT to allow GBT to sublease the Company’s portion of the building beginning in March 2015. For the three and nine months ended September 30, 2016, the Company recorded approximately $0.1 million and $0.3 million, respectively, of sublease income and $0.1 million and $0.3 million, respectively, of sublease expense, which is recorded in interest and other income, net in the consolidated statements of operations and comprehensive loss. On September 15, 2014, the Company entered into a five-year lease for approximately 34,400 square feet of office and laboratory space in South San Francisco. The Company may extend the lease for an additional three year term. The initial annual lease payments are $1.3 million, increasing to $1.6 million in the final year of the agreement. The lease period commenced in January 2015. The Company received a lease abatement for the first three months of the lease term, which is recorded as deferred rent and recognized over the lease term. The Company has provided deposits for letters of credit totaling $0.3 million to secure its obligations under its leases, which have been classified as long-term assets on the Company’s consolidated balance sheet as of September 30, 2016. Rent expense, net was $0.3 million and $1.0 million, and $0.4 million and $1.1 million, for the three and nine months ended September 30, 2016 and 2015, respectively. Contingencies From time to time, the Company may have contingent liabilities that arise in the ordinary course of business activities. The Company accrues for such a liability when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. There were no contingent liabilities requiring accrual or disclosure as of September 30, 2016, or December 31, 2015. Guarantees and Indemnifications The Company enters into standard indemnification arrangements in the ordinary course of business. Pursuant to certain of these arrangements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified parties for losses suffered or incurred by the indemnified party, in connection with any trade secret, copyright, patent or other intellectual property infringement claim by any third-party with respect to the Company’s technology. The term of these indemnification arrangements is generally perpetual. The maximum potential amount of future payments the Company could be required to make under these agreements is not determinable because it involves claims that may be made against the Company in the future, but have not yet been made. The Company indemnifies its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws, and agreements providing for indemnification entered into with its officers and directors. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification of directors and officers is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with its exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2016
Stockholders Equity Note [Abstract]
Stockholders' Equity	7. Stockholders’ Equity Common Stock Reserved for Issuance The Company has reserved shares of common stock for issuance as follows: September 30, 2016 December 31, 2015 Options issued and outstanding 2,034,777 1,318,647 Shares available for issuance under 2015 Stock Option and Incentive Plan 828,012 1,497,071 Shares available for issuance under 2015 Employee Stock Purchase Plan 229,971 255,000 Total 3,092,760 3,070,718 Redeemable Convertible Preferred Stock Prior to the IPO in October 2015, the Company had 62,235,313 shares of redeemable convertible preferred stock outstanding with a par value of $0.0001 per share, of which 38,500,000 were designated Series A redeemable convertible preferred stock, 6,666,667 were designated Series A-1 redeemable convertible preferred stock, and 17,068,646 were designated Series B redeemable convertible preferred stock. As disclosed in the Company’s Form 10-K for the year ended December 31, 2015, the Company accreted redeemable convertible preferred stock to redemption value and also accreted dividends in relation to these shares of redeemable convertible preferred stock. For the three and nine months ended September 30, 2015, the Company accreted redeemable convertible preferred stock to redemption value by $26,000 and $88,000, respectively, and accreted dividends on redeemable convertible preferred stock of $1.9 million and $4.5 million, respectively.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation The Company classifies stock-based compensation expense in the accompanying condensed consolidated statements of operations and comprehensive loss based on the department to which a recipient belongs. The following table sets forth stock-based compensation expense related to options granted to employees and consultants for all periods presented (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Research and development $ 269 $ 29 $ 690 $ 79 General and administrative 494 113 1,091 167 Total $ 763 $ 142 $ 1,781 $ 246 The following summarizes option activity under the 2012 Equity Incentive Plan and 2015 Stock Option and Incentive Plan: Shares Subject to Weighted Average Outstanding Options Exercise Price Per Share Balance at December 31, 2015 1,318,647 2.29 Options granted 786,034 10.02 Options exercised (18,913 ) 0.71 Options canceled (50,991 ) 5.19 Balance at September 30, 2016 2,034,777 5.22 Pursuant to the terms of the Company’s 2015 Employee Stock Purchase Plan (the “2015 ESPP”), on April 29, 2016, the Company issued 25,029 shares to participants in the 2015 ESPP in exchange for their contributions during the period from October 29, 2015 to April 29, 2016. In relation

Net Loss per Share Attributable

Net Loss per Share Attributable to Common Stockholders	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Net Loss per Share Attributable to Common Stockholders	9. Net Loss per Share Attributable to Common Stockholders The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Numerator Net loss $ (9,231 ) $ (5,462 ) $ (27,411 ) $ (15,037 ) Cumulative dividends on redeemable convertible preferred stock — (1,900 ) — (4,530 ) Accretion of redeemable convertible preferred stock to redemption value — (26 ) — (88 ) Net loss attributable to common stockholders, basic and diluted $ (9,231 ) $ (7,388 ) $ (27,411 ) $ (19,655 ) Denominator Weighted average shares outstanding 27,023,650 3,923,228 27,023,951 3,846,864 Less: weighted average shares subject to repurchase (553,352 ) (1,268,863 ) (692,099 ) (1,382,988 ) Weighted average shares used to compute basic and diluted net loss per share 26,470,298 2,654,365 26,331,852 2,463,876 Net loss per share attributable to common stockholders: Basic and diluted $ (0.35 ) $ (2.78 ) $ (1.04 ) $ (7.98 ) The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: September 30, 2016 2015 Common stock subject to repurchase 517,219 1,200,946 Stock options to purchase common stock 2,034,777 1,160,264 As of September 30, 2016, the Company has contributions from plan participants of $235,060 under the 2015 ESPP, which if converted, would be equivalent to 24,148 shares based on 85% of the stock price at the beginning of the offering period.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2016
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions In September 2012, the Company began receiving consulting and management services pursuant to an unwritten agreement with Third Rock Ventures, which is one of the Company’s largest shareholders. Charles Homcy and Kevin Starr, both directors of the Company, are venture partner and partner, respectively, of Third Rock Ventures. The consulting fees paid to Third Rock Ventures were incurred by the Company in the ordinary course of business, and were $7,000 and $31,000 for the three and nine months ended September 30, 2016 respectively, and $19,000 and $71,000 for the three and nine months ended September 30, 2015, respectively. As of September 30, 2016 and December 31, 2015, the Company had an outstanding liability to Third Rock Ventures of $12,000 and $13,000, respectively. Effective July 29, 2013, the Company entered into a shared facilities and services agreement with GBT, a company that was majority-owned by Third Rock Ventures as of such date. In connection with that agreement, the Company reimbursed GBT for shared facilities and equipment of the Company’s former corporate headquarters location. During the three and nine months ended September 30, 2016, the Company did not reimburse any expenses or equipment to GBT, and during the three and nine months ended September 30, 2015, the Company reimbursed expenses and equipment of zero and $41,000 respectively, to GBT. There was no outstanding liability to GBT as of September 30, 2016 and December 31, 2015. In October 2014, the Company entered into a lease assignment agreement with the owner of the former headquarters building and GBT to allow GBT to sublease the Company’s portion of the office after the Company relocated to its new corporate headquarters.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2016
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events On October 3, 2016, the Company completed a follow-on public offering of 4,370,000 shares of common stock at an offering price of $15.00 per share, resulting in net proceeds of approximately $61.0 million, after deducting underwriting discounts, commissions and estimated offering costs.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), the Company meets the definition of an emerging growth company, and has irrevocably elected to opt out of the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. In August 2016, the FASB issued Accounting Standard Update (“ASU”) No. 2016-15, Statement of Cash Flows (Topic 230) – Classification of Certain Cash Receipts and Cash Payments. In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts - Narrow-Scope Improvements and Practical Expedients. Revenue from Contracts with Customers In March 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts, Revenue Recognition. In March 2016, the FASB issued ASU No. 2016-09, Share-Based Compensation, In February 2016, the FASB issued ASU No. 2016-02 (Topic 842) Leases . ASU 2016-02 In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. In June 2014, the FASB issued ASU 2014-12, Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers Revenue Recognition.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Summary of Changes in Fair Value of Financial Instruments	The following table sets forth a summary of the changes in the fair value of the Company’s Level 1 financial instruments as follows (in thousands): Fair Value Measurements at September 30, 2016 Total Level 1 Level 2 Level 3 Assets Money Market funds $ 76,934 $ 76,934 $ — $ — Fair Value Measurements at December 31, 2015 Total Level 1 Level 2 Level 3 Assets Money Market funds $ 111,533 $ 111,533 $ — $ —
Summary of Changes in Fair Value of Company's Level 3 Financial Instruments	The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments as follows (in thousands): Convertible preferred stock call option liability, net Balance at December 31, 2014 $ 314 Change in fair value recorded in statement of operations and comprehensive loss (314 ) Balance at September 30, 2015 $ —

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2016
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): September 30, 2016 December 31, 2015 Scientific equipment $ 4,597 $ 3,995 Furniture and equipment 488 301 Capitalized software 231 225 Leasehold improvements 308 303 Total 5,624 4,824 Less: Accumulated depreciation and amortization (2,895 ) (2,080 ) Property and equipment, net $ 2,729 $ 2,744
Summary of Accrued Liabilities	Accrued liabilities consist of the following (in thousands): September 30, 2016 December 31, 2015 Payroll and related expenses $ 3,257 $ 2,515 Clinical research and development 2,618 2,145 Legal and accounting fees 409 33 Other 646 940 Total accrued liabilities $ 6,930 $ 5,633

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2016
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance	The Company has reserved shares of common stock for issuance as follows: September 30, 2016 December 31, 2015 Options issued and outstanding 2,034,777 1,318,647 Shares available for issuance under 2015 Stock Option and Incentive Plan 828,012 1,497,071 Shares available for issuance under 2015 Employee Stock Purchase Plan 229,971 255,000 Total 3,092,760 3,070,718

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense Related to Options Granted to Employees and Consultants	The following table sets forth stock-based compensation expense related to options granted to employees and consultants for all periods presented (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Research and development $ 269 $ 29 $ 690 $ 79 General and administrative 494 113 1,091 167 Total $ 763 $ 142 $ 1,781 $ 246
Summary of Stock Option Activity Under Plans	The following summarizes option activity under the 2012 Equity Incentive Plan and 2015 Stock Option and Incentive Plan: Shares Subject to Weighted Average Outstanding Options Exercise Price Per Share Balance at December 31, 2015 1,318,647 2.29 Options granted 786,034 10.02 Options exercised (18,913 ) 0.71 Options canceled (50,991 ) 5.19 Balance at September 30, 2016 2,034,777 5.22

Net Loss per Share Attributab22

Net Loss per Share Attributable to Common Stockholders (Tables)	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss per Share	The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Numerator Net loss $ (9,231 ) $ (5,462 ) $ (27,411 ) $ (15,037 ) Cumulative dividends on redeemable convertible preferred stock — (1,900 ) — (4,530 ) Accretion of redeemable convertible preferred stock to redemption value — (26 ) — (88 ) Net loss attributable to common stockholders, basic and diluted $ (9,231 ) $ (7,388 ) $ (27,411 ) $ (19,655 ) Denominator Weighted average shares outstanding 27,023,650 3,923,228 27,023,951 3,846,864 Less: weighted average shares subject to repurchase (553,352 ) (1,268,863 ) (692,099 ) (1,382,988 ) Weighted average shares used to compute basic and diluted net loss per share 26,470,298 2,654,365 26,331,852 2,463,876 Net loss per share attributable to common stockholders: Basic and diluted $ (0.35 ) $ (2.78 ) $ (1.04 ) $ (7.98 )
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss per Share	The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: September 30, 2016 2015 Common stock subject to repurchase 517,219 1,200,946 Stock options to purchase common stock 2,034,777 1,160,264

Formation and Business of the23

Formation and Business of the Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 03, 2016	Nov. 03, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit			$ (92,096)		$ (64,685)
Cash and cash equivalents			77,120	$ 64,545	$ 112,265	$ 43,648
Net proceeds from initial public offering		$ 55,600
Net proceeds from issuance of common stock			226	$ 352
Series A, A-1 and Series B Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from sale of shares of convertible stock			$ 93,900
IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock		6,253,125
Shares issued, price per share		$ 10
Follow-on Public Offering | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock	4,370,000
Shares issued, price per share	$ 15
Net proceeds from issuance of common stock	$ 61,000

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Changes in Fair Value of Financial Instruments (Details) - Fair Value Measurements on Recurring Basis - Money market funds - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Assets fair value	$ 76,934	$ 111,533
Level 1
Assets
Assets fair value	$ 76,934	$ 111,533

Fair Value Measurements - Sum25

Fair Value Measurements - Summary of Changes in Fair Value of Company's Level 3 Financial Instruments (Details) - Convertible Preferred Stock Call Option Liability, Net $ in Thousands	9 Months Ended
	Sep. 30, 2015USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 314
Change in fair value recorded in statement of operations and comprehensive loss	$ (314)

Collaboration and License Agr26

Collaboration and License Agreement - Additional Information (Details) - Sanofi (Aventis Inc.) - Collaborative Agreement	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 30, 2015USD ($)	Aug. 31, 2014USD ($)Program$ / sharesshares	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash received under collaboration agreement		$ 35,000,000
Initial equity investment		10,000,000
Eligible to receive milestone payment		$ 25,000,000
Period for research and clinical activities		4 years
Number of research programs | Program		3
Deferred revenue		$ 35,000,000	$ 3,500,000		$ 3,500,000		$ 14,200,000
Revenue recognized under collaboration agreement			3,600,000	$ 3,600,000	$ 10,600,000	$ 10,600,000
One-time non-refundable non-creditable cash payment		45,000,000
Project continuation, additional payment		40,000,000
Project continuation, additional payment description					an additional $40.0 million in connection with the purchase of the Company’s preferred stock, assuming (i) the Company had not previously closed either a Qualified IPO (at which time this obligation would terminate) or a private financing prior to a Qualified IPO and (ii) Sanofi had not previously purchased shares of the Company’s stock pursuant to such rights to purchase the Company’s capital stock in accordance with the terms of the Collaboration Agreement.
Time restricted rights to purchase stock, value					$ 40,000,000
Convertible preferred stock at discount			$ 40,000,000		$ 40,000,000
Redeemable convertible preferred stock call option liability fair value		$ 700,000
Change in fair value of redeemable convertible preferred stock call option liability							$ (300,000)	$ (400,000)
Series A-1 Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock | shares		6,666,667
Fair value of stock per share price | $ / shares		$ 1.50
Series B Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Project continuation, additional payment	$ 35,000,000
Decrease in project continuation additional payment	$ 5,000,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from collaboration agreement		$ 200,000,000
Project continuation payment		85,000,000
Funding from approved in-kind research and clinical activities		$ 45,000,000

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Property and equipment	$ 5,624	$ 4,824
Less: Accumulated depreciation and amortization	(2,895)	(2,080)
Property and equipment, net	2,729	2,744
Scientific Equipment
Property Plant And Equipment [Line Items]
Property and equipment	4,597	3,995
Furniture and Equipment
Property Plant And Equipment [Line Items]
Property and equipment	488	301
Capitalized Software
Property Plant And Equipment [Line Items]
Property and equipment	231	225
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	$ 308	$ 303

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization	$ 300,000	$ 200,000	$ 815,000	$ 741,000
Deferred offering costs capitalized	$ 500,000		$ 500,000		$ 0

Balance Sheet Components - Su29

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Balance Sheet Related Disclosures [Abstract]
Payroll and related expenses	$ 3,257	$ 2,515
Clinical research and development	2,618	2,145
Legal and accounting fees	409	33
Other	646	940
Total accrued liabilities	$ 6,930	$ 5,633

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	Sep. 15, 2014USD ($)ft²	Jun. 29, 2012USD ($)ft²	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)
Commitments And Contingencies Disclosure [Line Items]
Purchase commitment cancellation notice period					30 days
Rent expense net			$ 300,000	$ 400,000	$ 1,000,000	$ 1,100,000
Contingent liability for accrual			0		0		$ 0
Other Long Term Assets | Letter of Credit
Commitments And Contingencies Disclosure [Line Items]
Deposits for letter of credit			300,000		300,000
Facility Leases
Commitments And Contingencies Disclosure [Line Items]
Lease period	5 years	66 months
Area of leased property | ft²	34,400	12,000
Annual lease payments		$ 500,000
Sublease income			100,000		300,000
Sublease expense			$ 100,000		$ 300,000
Additional period of extension in lease contract	3 years
Initial annual lease payments	$ 1,300,000
Increase in lease payments at final year of agreement	$ 1,600,000
Lease commencement period					2015-01
Lease abatement period	3 months

Stockholders' Equity- Schedule

Stockholders' Equity- Schedule of Common Stock Reserved for Issuance(Details) - shares	Sep. 30, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	3,092,760	3,070,718
Options Issued and Outstanding
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	2,034,777	1,318,647
2015 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	828,012	1,497,071
2015 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	229,971	255,000

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Oct. 31, 2015
Class Of Stock [Line Items]
Accretion of redeemable convertible preferred stock to redemption value	$ 26	$ 88
Accretion of dividends on redeemable convertible preferred stock	$ 1,900	$ 4,530
Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Redeemable convertible preferred stock, shares outstanding			62,235,313
Redeemable convertible preferred stock, par value			$ 0.0001
Series A Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Redeemable convertible preferred stock, shares outstanding			38,500,000
Series A-1 Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Redeemable convertible preferred stock, shares outstanding			6,666,667
Series B Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Redeemable convertible preferred stock, shares outstanding			17,068,646

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense Related to Options Granted to Employees and Consultants (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 763	$ 142	$ 1,781	$ 246
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	269	29	690	79
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 494	$ 113	$ 1,091	$ 167

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity Under Plans (Details)	9 Months Ended
	Sep. 30, 2016$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Outstanding Options, Beginning Balance | shares	1,318,647
Shares Subject to Outstanding Options, granted | shares	786,034
Shares Subject to Outstanding Options, exercised | shares	(18,913)
Shares Subject to Outstanding Options, canceled | shares	(50,991)
Shares Subject to Outstanding Options, Ending Balance | shares	2,034,777
Weighted Average Exercise Price Per Share, Beginning Balance | $ / shares	$ 2.29
Weighted Average Exercise Price Per Share, granted | $ / shares	10.02
Weighted Average Exercise Price Per Share, exercised | $ / shares	0.71
Weighted Average Exercise Price Per Share, cancelled | $ / shares	5.19
Weighted Average Exercise Price Per Share, Ending Balance | $ / shares	$ 5.22

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	Apr. 29, 2016	Sep. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized share based compensation expense		$ 147,000
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued to participants exchange	25,029

Net Loss per Share Attributab36

Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator
Net loss	$ (9,231)	$ (5,462)	$ (27,411)	$ (15,037)
Cumulative dividends on redeemable convertible preferred stock		(1,900)		(4,530)
Accretion of redeemable convertible preferred stock to redemption value		(26)		(88)
Net loss attributable to common stockholders	$ (9,231)	$ (7,388)	$ (27,411)	$ (19,655)
Denominator
Weighted average shares outstanding	27,023,650	3,923,228	27,023,951	3,846,864
Less: weighted average shares subject to repurchase	(553,352)	(1,268,863)	(692,099)	(1,382,988)
Weighted average shares used to compute basic and diluted net loss per share	26,470,298	2,654,365	26,331,852	2,463,876
Net loss per share attributable to common stockholders:
Basic and diluted	$ (0.35)	$ (2.78)	$ (1.04)	$ (7.98)

Net Loss per Share Attributab37

Net Loss per Share Attributable to Common Stockholders - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Common stock subject to repurchase
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	517,219	1,200,946
Stock options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	2,034,777	1,160,264

Net Loss per Share Attributab38

Net Loss per Share Attributable to Common Stockholders - Additional Information (Details) - 2015 ESPP	9 Months Ended
	Sep. 30, 2016USD ($)shares
Earnings Per Share [Line Items]
ESPP contributions from plan participants | $	$ 235,060
Contributions converted shares under benefit plan | shares	24,148
Percentage of stock price at the beginning of offering period	85.00%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Third Rock Ventures
Related Party Transaction [Line Items]
Related party costs	$ 7,000	$ 19,000	$ 31,000	$ 71,000
Due to related parties	12,000		12,000		$ 13,000
GBT
Related Party Transaction [Line Items]
Due to related parties	0		0		$ 0
Company reimbursed expenses and equipment	$ 0	$ 0	$ 0	$ 41,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 03, 2016	Sep. 30, 2016	Sep. 30, 2015
Subsequent Event [Line Items]
Net proceeds from issuance of common stock		$ 226	$ 352
Follow-on Public Offering | Subsequent Event
Subsequent Event [Line Items]
Issuance of common stock	4,370,000
Shares issued, price per share	$ 15
Net proceeds from issuance of common stock	$ 61,000