MyoKardia (MYOK) 10-Q 3 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 31, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
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	Accelerated Filer
Entity Common Stock, Shares Outstanding		35,741,036

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 227,184	$ 135,797
Short-term investments	19,993	4,072
Receivable from collaboration partner		45,000
Prepaid expenses and other current assets	1,545	1,394
Total current assets	248,722	186,263
Property and equipment, net	2,633	2,758
Long-term investments	36,003	12,002
Other long-term assets	432	283
Total assets	287,790	201,306
Current liabilities
Accounts payable	2,030	1,798
Accrued liabilities	10,634	8,690
Deferred revenue - current	22,500	22,500
Total current liabilities	35,164	32,988
Other long-term liabilities	246	436
Deferred revenue - noncurrent	5,625	22,500
Total liabilities	41,035	55,924
Commitments and contingencies (Note 6)
Stockholders’ equity
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; none issued and outstanding
Common stock, $0.0001 par value, 150,000,000 and 150,000,000 shares authorized at September 30, 2017 and December 31, 2016, respectively; 35,733,002 and 31,428,998 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	4	3
Additional paid-in capital	363,205	223,208
Accumulated other comprehensive (loss) income	(52)	8
Accumulated deficit	(116,402)	(77,837)
Total stockholders’ equity	246,755	145,382
Total liabilities and stockholders’ equity	$ 287,790	$ 201,306

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
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	35,733,002	31,428,998
Common stock, shares outstanding	35,733,002	31,428,998

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Collaboration and license revenue	$ 5,625	$ 3,550	$ 16,875	$ 10,649
Operating expenses:
Research and development	14,361	8,783	39,967	26,192
General and administrative	5,884	4,031	16,442	11,947
Total operating expenses	20,245	12,814	56,409	38,139
Loss from operations	(14,620)	(9,264)	(39,534)	(27,490)
Interest and other income, net	447	33	977	79
Net loss	(14,173)	(9,231)	(38,557)	(27,411)
Other comprehensive loss	2		60
Comprehensive loss	(14,171)	(9,231)	(38,497)	(27,411)
Net loss attributable to common stockholders	$ (14,173)	$ (9,231)	$ (38,557)	$ (27,411)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.42)	$ (0.35)	$ (1.21)	$ (1.04)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted	33,525,567	26,470,298	31,951,631	26,331,852

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (38,557)	$ (27,411)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,347	1,781
Depreciation	977	815
Accretion of discounts and amortization of premiums on investments	62
Change in operating assets and liabilities:
Receivable from collaboration partner	45,000
Prepaid expenses and other current assets	(151)	520
Other long-term assets	(149)
Accounts payable	228	(372)
Accrued liabilities	2,029	1,027
Other long-term liabilities	(118)	(75)
Deferred revenue	(16,875)	(10,649)
Net cash used in operating activities	(3,207)	(34,364)
Cash flows from investing activities:
Purchases of investments	(44,044)
Sale of investments	4,000
Purchases of property and equipment	(917)	(854)
Net cash used in investing activities	(40,961)	(854)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of offering costs	133,925
Proceeds from exercise of stock options and employee stock purchase plan	1,668	226
Payments of prior period offering costs	(38)	(153)
Net cash provided by financing activities	135,555	73
Net increase (decrease) in cash and cash equivalents	91,387	(35,145)
Cash and cash equivalents, beginning of period	135,797	112,265
Cash and cash equivalents, end of period	227,184	77,120
Supplemental disclosures of noncash investing and financing activities
Unpaid offering costs included in period-end accounts payable and accrued liabilities	63	461
Vesting of early exercised options and restricted stock	156	226
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 38	$ 7

Formation and Business of the C

Formation and Business of the Company	9 Months Ended
	Sep. 30, 2017
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”) 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 (“HCM”), and dilated cardiomyopathy (“DCM”). The Company has completed enrollment in a Phase 2 clinical trial of mavacamten (formerly known as MYK-461), our product candidate for the treatment of HCM, and is currently enrolling subjects in a Phase 1 clinical trial of MYK-491, our product candidate for the treatment of DCM. Using our precision medicine development strategy, we believe we have efficiently generated clinical proof of mechanism for mavacamten in both healthy volunteers and in HCM patients, and we intend to pursue a similar path for MYK-491. In 2016, mavacamten was granted Orphan Drug Designation by the U.S. Food and Drug Administration (“FDA”), for the treatment of symptomatic, obstructive hypertrophic cardiomyopathy (“oHCM”), a subset of HCM. Through September 30, 2017, the Company has financed its operations through an initial public offering (“IPO”), two follow-on public offerings, 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 $61.1 million, after deducting underwriting discounts, commissions and estimated offering costs. On August 14, 2017, the Company completed another follow-on public offering of 4,025,000 shares of common stock at an offering price of $35.50 per share, resulting in net proceeds of approximately $133.8 million, after deducting underwriting discounts, commissions and estimated offering costs. In connection with the Collaboration Agreement, the Company has received $105.0 million from Sanofi S.A., consisting of a $35.0 million upfront payment, a $25.0 million milestone payment for the submission of an investigational new drug application (“IND”) for MYK-491 with the FDA in November 2016, and a $45.0 million continuation payment from Sanofi in January 2017. As of September 30, 2017, the Company had an accumulated deficit of $116.4 million and cash and cash equivalents of $227.2 million, short-term investments of $20.0 million and long-term investments of $36.0 million. 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, 2016 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, 2017, 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, 2016 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016 filed with the U.S. Securities and Exchange Commission (the “SEC”) on March 13, 2017 (the “Annual Report”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
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, 2016 included in the Annual Report. There have been no changes to the Company’s significant accounting policies during the nine months ended September 30, 2017, except as described below. Adopted Accounting Pronouncements Beginning in fiscal year 2017, the Company adopted Accounting Standard Update (“ASU”) No. 2016-09, Improvements to employee share-based payment accounting, 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 May 2017, the FASB issued ASU 2017-09—Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting. The amendments in this ASU provide guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718 to address diversity in practice. An entity should account for the effects of a modification unless all the three specified conditions are met. The current disclosure requirements in Topic 718 apply regardless of whether an entity is required to apply modification accounting under the amendments in this ASU. The amendments in this ASU are effective for all entities for annual periods, and interim periods within those annual periods, beginning after December 15, 2017, with early adoption permitted. The amendments in this ASU should be applied prospectively to an award modified on or after the adoption date. In November 2016, the FASB issued ASU No. 2016-18 (Topic 230), Restricted Cash, Statement of Cash Flows ment of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 is effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years, with early adoption permitted. The amendments in this ASU should be applied using a retrospective transition method to each period presented. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements. In February ASU No. 2016-02 (Topic 842), Leases. requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, with early adoption permitted. In May 2014, the FASB issued ASU No. 2014-09 (Topic 606), Revenue from Contracts with Customers ASC 605, Revenue Recognition.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2017
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 the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): Fair Value Measurements at September 30, 2017 Total Level 1 Level 2 Level 3 Assets Money market funds $ 227,466 $ 227,466 $ — $ — U.S. government agency obligations 31,963 — 31,963 — Corporate securities 24,033 — 24,033 — Total $ 283,462 $ 227,466 $ 55,996 $ — Fair Value Measurements at December 31, 2016 Total Level 1 Level 2 Level 3 Assets Money market funds $ 136,481 $ 136,481 $ — $ — U.S. government agency obligations 12,075 — 12,075 — Corporate securities 3,999 — 3,999 — Total $ 152,555 $ 136,481 $ 16,074 $ — The following table is a summary of amortized cost, unrealized gain and loss, and fair value (in thousands) of the Company’s marketable securities by contractual maturities: Fair Value Measurements at September 30, 2017 Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 227,466 $ — $ — $ 227,466 Short-term investments (due within one year) 20,009 — (16 ) $ 19,993 Long-term investments (due between one and two years) 36,033 3 (33 ) $ 36,003 Total $ 283,508 $ 3 $ (49 ) $ 283,462 Fair Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 136,481 $ — $ — $ 136,481 Short-term investments (due within one year) 4,072 — — 4,072 Long-term investments (due between one and two years) 11,988 14 — 12,002 Total $ 152,541 $ 14 $ — $ 152,555

Collaboration and License Agree

Collaboration and License Agreement	9 Months Ended
	Sep. 30, 2017
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., 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 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. In addition, under the terms of the Collaboration Agreement, Sanofi may reimburse the Company for a portion of the registration program costs for mavacamten. These registration costs will be reimbursed under the Registration Program Plan approved in October 2017. The Collaboration Agreement covers three main research programs, “HCM1” (or HCM-1 or mavacamten, formerly known as MYK-461), “HCM2” (or HCM-2) and “DCM1” (or 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 mavacamten 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 mavacamten and HCM-2 programs where it has ex-United States commercialization rights. Sanofi also has the option to co-promote in the U.S. for potential expanded cardiovascular diseases outside of the genetically targeted indications for the mavacamten 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 was recognizing as revenue on a straight-line basis over the expected term of research and development services through December 31, 2016 because there was not a more discernible pattern of performance in which the research and development services occurred. During the three months ended September 30, 2017 and 2016, the Company recognized zero and $3.6 million, respectively, and during the nine months ended September 30, 2017 and 2016, the Company recognized zero and $10.6 million of revenue, respectively, related to the $35.0 million upfront payment under the Collaboration Agreement. As of September 30, 2017 and December 31, 2016, the Company did not have any deferred revenue on its consolidated balance sheet related to this upfront payment. 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 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. As of September 30, 2017, Sanofi owned 11.2% of the Company’s common stock. 3. $25.0 million milestone-based payment. The Company was 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 accounted for this milestone payment separately from the rest of the agreement. The Company has determined that the milestone was substantive as it was achieved based upon the Company’s past performance. The Company achieved this milestone in October 2016 and as a result, recognized the $25.0 million milestone payment from Sanofi as revenue during the year ended December 31, 2016. 4. Up to $85.0 million continuation payments. Under the Collaboration Agreement, Sanofi needed to determine by December 31, 2016 whether or not to continue the Collaboration Agreement. Under the terms of the Collaboration Agreement, if Sanofi so elected to continue the Collaboration Agreement, it would be obligated to pay: • a one-time, non-refundable, non-creditable cash payment of $45.0 million; and • an additional $40.0 million reduced by $5.0 million in connection with the purchase of the Company’s preferred stock, assuming the Company has not previously closed (i) either a Qualified IPO (at which time this obligation will terminate) or a private financing prior to a Qualified IPO and (ii) Sanofi has 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 elected to continue the Collaboration Agreement in December 2016. The Company recorded a receivable and deferred revenue as of December 31, 2016 for the $45.0 million continuation payment, upon receipt of the election to continue, which the Company is recognizing on a straight-line basis over the expected term of research and development services through December 31, 2018 because there is no more discernable pattern of performance for which the R&D services occur. The payment was subsequently received in January 2017. In relation to this continuation payment, the Company recognized $5.6 million and $16.9 million as revenue during the three and nine months ended September 30, 2017, respectively, and had deferred revenue on its consolidated balance sheet of $28.1 million as of September 30, 2017. 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 had 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 did not have a liability related to the redeemable convertible preferred stock call option on its consolidated balance sheet as of September 30, 2017 and December 31, 2016. 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, 2017
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, 2017 December 31, 2016 Scientific equipment $ 5,531 $ 4,858 Furniture and equipment 682 546 Capitalized software 280 237 Leasehold improvements 308 308 Total 6,801 5,949 Less: Accumulated depreciation (4,168 ) (3,191 ) Property and equipment, net $ 2,633 $ 2,758 Depreciation expense was $0.4 million and $0.3 million, for the three months ended September 30, 2017 and 2016, respectively, and $1.0 million and $0.8 million, for the nine months ended September 30, 2017 and 2016, respectively. Accrued Liabilities Accrued liabilities consist of the following (in thousands): September 30, 2017 December 31, 2016 Clinical research and development $ 5,318 $ 3,981 Payroll and related expenses 3,919 3,717 Other 1,397 992 Total accrued liabilities $ 10,634 $ 8,690

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2017
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. 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, 2017, 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. 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, 2017. Rent expense, net, was $0.3 million and $1.0 million, for each of the three and nine months ended September 30, 2017 and 2016, 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, 2017, or December 31, 2016. 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, 2017
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, 2017 December 31, 2016 Options issued and outstanding 3,289,677 2,141,868 Shares available for issuance under 2015 Stock Option and Incentive Plan 587,696 720,921 Shares available for issuance under 2015 Employee Stock Purchase Plan 479,947 202,087 Total 4,357,320 3,064,876

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2017
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 September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 713 $ 269 $ 1,913 $ 690 General and administrative 888 494 2,434 1,091 Total $ 1,601 $ 763 $ 4,347 $ 1,781 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, 2016 2,141,868 $ 6.42 Options granted 1,685,575 14.69 Options exercised (282,978 ) 4.46 Options canceled (254,788 ) 9.13 Balance at September 30, 2017 3,289,677 $ 10.62 In relation $147,000 . The Company begins to recognize expenses related to these options during the period upon concluding that certain performance criteria are considered probable.

Net Loss per Share Attributable

Net Loss per Share Attributable to Common Stockholders	9 Months Ended
	Sep. 30, 2017
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, 2017 2016 2017 2016 Numerator Net loss $ (14,173 ) $ (9,231 ) $ (38,557 ) $ (27,411 ) Net loss attributable to common stockholders, basic and diluted $ (14,173 ) $ (9,231 ) $ (38,557 ) $ (27,411 ) Denominator Weighted average shares outstanding 33,686,799 27,023,650 32,195,471 27,023,951 Less: weighted average shares subject to repurchase (161,232 ) (553,352 ) (243,840 ) (692,099 ) Weighted average shares used to compute basic and diluted net loss per share 33,525,567 26,470,298 31,951,631 26,331,852 Net loss per share attributable to common stockholders, basic and diluted $ (0.42 ) $ (0.35 ) $ (1.21 ) $ (1.04 ) 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: As of September 30, 2017 2016 Common stock subject to repurchase 127,524 517,219 Stock options to purchase common stock 3,289,677 2,034,777 As of September 30, 2017, the Company has contributions from plan participants of $297,000 under the 2015 ESPP, which if converted, would be equivalent to 25,668 shares based on 85% of the stock price at the beginning of the offering period. As of September 30, 2016, the Company had 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, 2017
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. Kevin Starr, a director of the Company, is a partner of Third Rock Ventures. Charles Homcy, a former director who resigned from the Board of Directors in March 2017, is a venture partner 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 $12,000 and $45,000 for the three and nine months ended September 30, 2017, respectively, and $7,000 and $31,000 for the three and nine months ended September 30, 2016, respectively. As of September 30, 2017 and December 31, 2016, the Company had outstanding obligations to Third Rock Ventures of $13,000 and $9,000, respectively.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events The Company currently leases approximately 34,400 square feet of laboratory and office space in South San Francisco, California under a lease that expires on January 19, 2020. On October 1, 2017, the Company entered into an additional 25-month sublease agreement for approximately 8,000 square feet of office space in South San Francisco with annual payments of approximately $0.3 million. The lease period commenced on October 1, 2017.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Adopted Accounting Pronouncements	Adopted Accounting Pronouncements Beginning in fiscal year 2017, the Company adopted Accounting Standard Update (“ASU”) No. 2016-09, Improvements to employee share-based payment accounting,
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 May 2017, the FASB issued ASU 2017-09—Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting. The amendments in this ASU provide guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718 to address diversity in practice. An entity should account for the effects of a modification unless all the three specified conditions are met. The current disclosure requirements in Topic 718 apply regardless of whether an entity is required to apply modification accounting under the amendments in this ASU. The amendments in this ASU are effective for all entities for annual periods, and interim periods within those annual periods, beginning after December 15, 2017, with early adoption permitted. The amendments in this ASU should be applied prospectively to an award modified on or after the adoption date. In November 2016, the FASB issued ASU No. 2016-18 (Topic 230), Restricted Cash, Statement of Cash Flows ment of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 is effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years, with early adoption permitted. The amendments in this ASU should be applied using a retrospective transition method to each period presented. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements. In February ASU No. 2016-02 (Topic 842), Leases. requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, with early adoption permitted. In May 2014, the FASB issued ASU No. 2014-09 (Topic 606), Revenue from Contracts with Customers ASC 605, Revenue Recognition.
Fair Value Measurements	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.
Contingencies	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.
Guarantees and Indemnifications	Guarantees and Indemnifications 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 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.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Summary of Fair Value Assets and Liabilities Measured on Recurring Basis	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): Fair Value Measurements at September 30, 2017 Total Level 1 Level 2 Level 3 Assets Money market funds $ 227,466 $ 227,466 $ — $ — U.S. government agency obligations 31,963 — 31,963 — Corporate securities 24,033 — 24,033 — Total $ 283,462 $ 227,466 $ 55,996 $ — Fair Value Measurements at December 31, 2016 Total Level 1 Level 2 Level 3 Assets Money market funds $ 136,481 $ 136,481 $ — $ — U.S. government agency obligations 12,075 — 12,075 — Corporate securities 3,999 — 3,999 — Total $ 152,555 $ 136,481 $ 16,074 $ —
Summary of Fair Value Measurement of Available-for-sale Securities	The following table is a summary of amortized cost, unrealized gain and loss, and fair value (in thousands) of the Company’s marketable securities by contractual maturities: Fair Value Measurements at September 30, 2017 Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 227,466 $ — $ — $ 227,466 Short-term investments (due within one year) 20,009 — (16 ) $ 19,993 Long-term investments (due between one and two years) 36,033 3 (33 ) $ 36,003 Total $ 283,508 $ 3 $ (49 ) $ 283,462 Fair Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 136,481 $ — $ — $ 136,481 Short-term investments (due within one year) 4,072 — — 4,072 Long-term investments (due between one and two years) 11,988 14 — 12,002 Total $ 152,541 $ 14 $ — $ 152,555

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2017
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): September 30, 2017 December 31, 2016 Scientific equipment $ 5,531 $ 4,858 Furniture and equipment 682 546 Capitalized software 280 237 Leasehold improvements 308 308 Total 6,801 5,949 Less: Accumulated depreciation (4,168 ) (3,191 ) Property and equipment, net $ 2,633 $ 2,758
Summary of Accrued Liabilities	Accrued liabilities consist of the following (in thousands): September 30, 2017 December 31, 2016 Clinical research and development $ 5,318 $ 3,981 Payroll and related expenses 3,919 3,717 Other 1,397 992 Total accrued liabilities $ 10,634 $ 8,690

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance	The Company has reserved shares of common stock for issuance as follows: September 30, 2017 December 31, 2016 Options issued and outstanding 3,289,677 2,141,868 Shares available for issuance under 2015 Stock Option and Incentive Plan 587,696 720,921 Shares available for issuance under 2015 Employee Stock Purchase Plan 479,947 202,087 Total 4,357,320 3,064,876

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2017
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 September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 713 $ 269 $ 1,913 $ 690 General and administrative 888 494 2,434 1,091 Total $ 1,601 $ 763 $ 4,347 $ 1,781
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, 2016 2,141,868 $ 6.42 Options granted 1,685,575 14.69 Options exercised (282,978 ) 4.46 Options canceled (254,788 ) 9.13 Balance at September 30, 2017 3,289,677 $ 10.62

Net Loss per Share Attributab22

Net Loss per Share Attributable to Common Stockholders (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 2016 2017 2016 Numerator Net loss $ (14,173 ) $ (9,231 ) $ (38,557 ) $ (27,411 ) Net loss attributable to common stockholders, basic and diluted $ (14,173 ) $ (9,231 ) $ (38,557 ) $ (27,411 ) Denominator Weighted average shares outstanding 33,686,799 27,023,650 32,195,471 27,023,951 Less: weighted average shares subject to repurchase (161,232 ) (553,352 ) (243,840 ) (692,099 ) Weighted average shares used to compute basic and diluted net loss per share 33,525,567 26,470,298 31,951,631 26,331,852 Net loss per share attributable to common stockholders, basic and diluted $ (0.42 ) $ (0.35 ) $ (1.21 ) $ (1.04 )
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: As of September 30, 2017 2016 Common stock subject to repurchase 127,524 517,219 Stock options to purchase common stock 3,289,677 2,034,777

Formation and Business of the23

Formation and Business of the Company - Additional Information (Details) $ / shares in Units, $ in Thousands	Aug. 14, 2017USD ($)$ / sharesshares	Oct. 03, 2016USD ($)$ / sharesshares	Nov. 03, 2015USD ($)$ / sharesshares	Jan. 31, 2017USD ($)	Nov. 30, 2016USD ($)	Aug. 31, 2014USD ($)	Sep. 30, 2017USD ($)Offering	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2015USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of follow-on public offerings | Offering							2
Net proceeds from issuance of common stock							$ 133,925
Accumulated deficit							(116,402)	$ (77,837)
Cash and cash equivalents							227,184	135,797	$ 77,120	$ 112,265
Short-term investments							19,993	4,072
Long-term investments							36,003	12,002
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 | shares			6,253,125
Shares issued, price per share | $ / shares			$ 10
Net proceeds from initial public offering			$ 55,600
Follow-on Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock | shares	4,025,000	4,370,000
Shares issued, price per share | $ / shares	$ 35.50	$ 15
Net proceeds from issuance of common stock	$ 133,800	$ 61,100
Sanofi (Aventis Inc.) | Collaborative Agreement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Collaboration agreement, amount received					$ 105,000
Upfront cash received under collaboration agreement					35,000	$ 35,000
Revenue recognized under milestone-based payments					$ 25,000			$ 25,000
Continuation payment, amount received				$ 45,000

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Assets fair value	$ 283,462	$ 152,555
Money market funds
Assets
Assets fair value	227,466	136,481
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	227,466	136,481
Fair Value Measurements on Recurring Basis | Level 1 | Money market funds
Assets
Assets fair value	227,466	136,481
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	55,996	16,074
U.S. government agency obligations
Assets
Assets fair value	31,963	12,075
U.S. government agency obligations | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	31,963	12,075
Corporate securities
Assets
Assets fair value	24,033	3,999
Corporate securities | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	$ 24,033	$ 3,999

Fair Value Measurements - Sum25

Fair Value Measurements - Summary of Fair Value Measurement of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 283,508	$ 152,541
Unrealized Gain	3	14
Unrealized Loss	(49)
Fair Value	283,462	152,555
Cash equivalents (due within 90 days)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	227,466	136,481
Fair Value	227,466	136,481
Short-term investments (due within one year)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	20,009	4,072
Unrealized Loss	(16)
Fair Value	19,993	4,072
Long-term investments (due between one and two years)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	36,033	11,988
Unrealized Gain	3	14
Unrealized Loss	(33)
Fair Value	$ 36,003	$ 12,002

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
	Nov. 30, 2016USD ($)	Apr. 30, 2015USD ($)	Aug. 31, 2014USD ($)Program$ / sharesshares	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash received under collaboration agreement	$ 35,000,000		$ 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	$ 0		$ 0		$ 0
Revenue recognized under collaboration agreement				0	$ 3,600,000	$ 0	$ 10,600,000
Revenue recognized under milestone-based payments	$ 25,000,000							25,000,000
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 reduced by $5.0 million in connection with the purchase of the Company’s preferred stock, assuming the Company has not previously closed (i) either a Qualified IPO (at which time this obligation will terminate) or a private financing prior to a Qualified IPO and (ii) Sanofi has 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
Project continuation, revenue recognized				5,600,000		16,900,000
Project continuation, deferred revenue				28,100,000		28,100,000
Redeemable convertible preferred stock call option liability fair value			$ 700,000
Redeemable convertible preferred stock call option liability				$ 0		$ 0		$ 0
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
Common stock owned percentage				11.20%		11.20%
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, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment	$ 6,801	$ 5,949
Less: Accumulated depreciation	(4,168)	(3,191)
Property and equipment, net	2,633	2,758
Scientific Equipment
Property Plant And Equipment [Line Items]
Property and equipment	5,531	4,858
Furniture and Equipment
Property Plant And Equipment [Line Items]
Property and equipment	682	546
Capitalized Software
Property Plant And Equipment [Line Items]
Property and equipment	280	237
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	$ 308	$ 308

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Balance Sheet Related Disclosures [Abstract]
Depreciation expense	$ 400	$ 300	$ 977	$ 815

Balance Sheet Components - Su29

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Clinical research and development	$ 5,318	$ 3,981
Payroll and related expenses	3,919	3,717
Other	1,397	992
Total accrued liabilities	$ 10,634	$ 8,690

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	Sep. 15, 2014USD ($)ft²	Jun. 29, 2012USD ($)ft²	Sep. 30, 2017USD ($)ft²	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)ft²	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)
Commitments And Contingencies Disclosure [Line Items]
Purchase commitment cancellation notice period					30 days
Deposits for letter of credit			$ 300,000		$ 300,000
Rent expense net			300,000	$ 1,000,000	300,000	$ 1,000,000
Contingent liability for accrual			$ 0		$ 0		$ 0
Facility Leases
Commitments And Contingencies Disclosure [Line Items]
Lease period	5 years	66 months
Area of leased property | ft²	34,400	12,000	34,400		34,400
Annual lease payments		$ 500,000
Sublease income			$ 100,000	100,000	$ 300,000	300,000
Sublease expense			$ 100,000	$ 100,000	$ 300,000	$ 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, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	4,357,320	3,064,876
Options Issued and Outstanding
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	3,289,677	2,141,868
2015 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	587,696	720,921
2015 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	479,947	202,087

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,601	$ 763	$ 4,347	$ 1,781
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	713	269	1,913	690
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 888	$ 494	$ 2,434	$ 1,091

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity Under Plans (Details)	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Outstanding Options, Beginning Balance | shares	2,141,868
Shares Subject to Outstanding Options, granted | shares	1,685,575
Shares Subject to Outstanding Options, exercised | shares	(282,978)
Shares Subject to Outstanding Options, canceled | shares	(254,788)
Shares Subject to Outstanding Options, Ending Balance | shares	3,289,677
Weighted Average Exercise Price Per Share, Beginning Balance | $ / shares	$ 6.42
Weighted Average Exercise Price Per Share, granted | $ / shares	14.69
Weighted Average Exercise Price Per Share, exercised | $ / shares	4.46
Weighted Average Exercise Price Per Share, cancelled | $ / shares	9.13
Weighted Average Exercise Price Per Share, Ending Balance | $ / shares	$ 10.62

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense for nonvested options	$ 0	$ 74,000	$ 174,000	$ 147,000

Net Loss per Share Attributab35

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net loss	$ (14,173)	$ (9,231)	$ (38,557)	$ (27,411)
Net loss attributable to common stockholders	$ (14,173)	$ (9,231)	$ (38,557)	$ (27,411)
Denominator
Weighted average shares outstanding	33,686,799	27,023,650	32,195,471	27,023,951
Less: weighted average shares subject to repurchase	(161,232)	(553,352)	(243,840)	(692,099)
Weighted average shares used to compute basic and diluted net loss per share	33,525,567	26,470,298	31,951,631	26,331,852
Net loss per share attributable to common stockholders, basic and diluted	$ (0.42)	$ (0.35)	$ (1.21)	$ (1.04)

Net Loss per Share Attributab36

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, 2017	Sep. 30, 2016
Common stock subject to repurchase
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	127,524	517,219
Stock options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	3,289,677	2,034,777

Net Loss per Share Attributab37

Net Loss per Share Attributable to Common Stockholders - Additional Information (Details) - 2015 ESPP - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Line Items]
ESPP contributions from plan participants	$ 297,000	$ 235,060
Contributions converted shares under benefit plan	25,668	24,148
Percentage of stock price at the beginning of offering period	85.00%	85.00%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - Third Rock Ventures - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Related Party Transaction [Line Items]
Related party costs	$ 12,000	$ 7,000	$ 45,000	$ 31,000
Due to related parties	$ 13,000		$ 13,000		$ 9,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) $ in Millions	Oct. 01, 2017USD ($)ft²	Sep. 15, 2014ft²	Jun. 29, 2012USD ($)ft²	Sep. 30, 2017ft²
Facility Leases
Subsequent Event [Line Items]
Area of leased property | ft²		34,400	12,000	34,400
Lease expiration date				Jan. 19, 2020
Lease period		5 years	66 months
Annual lease payments | $			$ 0.5
Sublease Agreement | Subsequent Event
Subsequent Event [Line Items]
Area of leased property | ft²	8,000
Lease period	25 months
Annual lease payments | $	$ 0.3
Lease commencement period	Oct. 1, 2017