MyoKardia (MYOK) 10-Q 7 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 01, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	MYOK
Entity Registrant Name	MyoKardia Inc
Entity Central Index Key	0001552451
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		46,117,975
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-37609
Entity Tax Identification Number	445500552
Entity Address, Address Line One	333 Allerton Ave
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	741-0900

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 443,693	$ 246,122
Short-term investments	114,792	68,564
Prepaid expenses and other current assets	4,316	4,760
Total current assets	562,801	319,446
Property and equipment, net	5,435	5,138
Operating lease right-of-use assets	1,756
Long-term investments	43,952	80,148
Restricted cash and other	2,109	2,521
Total assets	616,053	407,253
Current liabilities
Accounts payable	3,037	2,946
Accrued liabilities	24,744	20,758
Prepayment from collaboration partner	2,256	12,973
Operating lease liabilities - current	1,831
Total current liabilities	31,868	36,677
Other long-term liabilities		9
Total liabilities	31,868	36,686
Commitments and contingencies (Note 7)
Stockholders’ equity
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; none issued and outstanding	0	0
Common stock, $0.0001 par value, 150,000,000 shares authorized at June 30, 2019 and December 31, 2018; 46,098,059 and 40,288,949 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	5	4
Additional paid-in capital	861,880	573,183
Accumulated other comprehensive income (loss)	497	(67)
Accumulated deficit	(278,197)	(202,553)
Total stockholders’ equity	584,185	370,567
Total liabilities and stockholders’ equity	$ 616,053	$ 407,253

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
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	46,098,059	40,288,949
Common stock, shares outstanding	46,098,059	40,288,949

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Collaboration and license revenue		$ 6,639		$ 11,970
Operating expenses:
Research and development	$ 27,708	17,218	$ 53,898	33,836
Selling, general and administrative	13,856	8,912	27,407	16,225
Total operating expenses	41,564	26,130	81,305	50,061
Loss from operations	(41,564)	(19,491)	(81,305)	(38,091)
Interest and other income, net	3,172	1,078	5,443	1,858
Loss before income taxes	(38,392)	(18,413)	(75,862)	(36,233)
Income tax benefit	(218)	0	(218)	0
Net loss	(38,174)	(18,413)	(75,644)	(36,233)
Other comprehensive income (loss), net of tax effect of $219, $0, $219, $0, respectively	201	70	564	(67)
Comprehensive loss	$ (37,973)	$ (18,343)	$ (75,080)	$ (36,300)
Net loss per share, basic and diluted	$ (0.83)	$ (0.49)	$ (1.75)	$ (0.99)
Weighted average number of shares used to compute net loss per share, basic and diluted	46,065,901	37,440,024	43,301,417	36,620,747

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Other comprehensive income (loss), tax	$ 219	$ 0	$ 219	$ 0

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Follow-on Offering	Common Stock	Common StockFollow-on Offering	Additional Paid-In Capital	Additional Paid-In CapitalFollow-on Offering	Accumulated Other Comprehensive Income/ (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 230,676		$ 4		$ 365,719		$ (192)	$ (134,855)
Beginning balance, shares at Dec. 31, 2017			35,812,791
Issuance of common stock upon the exercise of options and release of stock awards	806				806
Issuance of common stock upon the exercise of options and release of stock awards, shares			110,961
Repurchase of early exercised stock options, value	(1)				(1)
Repurchase of early exercised stock options, shares			(770)
Vesting of early exercised stock options	18				18
Stock-based compensation	3,631				3,631
Unrealized gains (losses), net of tax expense, (benefits)	(137)						(137)
Net loss	(17,820)							(17,820)
Ending balance at Mar. 31, 2018	217,173		$ 4		370,173		(329)	(152,675)
Ending balance, shares at Mar. 31, 2018			35,922,982
Beginning balance at Dec. 31, 2017	230,676		$ 4		365,719		(192)	(134,855)
Beginning balance, shares at Dec. 31, 2017			35,812,791
Net loss	(36,233)
Ending balance at Jun. 30, 2018	387,033		$ 4		558,376		(259)	(171,088)
Ending balance, shares at Jun. 30, 2018			40,049,160
Beginning balance at Mar. 31, 2018	217,173		$ 4		370,173		(329)	(152,675)
Beginning balance, shares at Mar. 31, 2018			35,922,982
Issuance of common stock during offering, net of issuance costs, value	181,863				181,863
Issuance of common stock during offering, net of issuance costs, shares			3,961,147
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, value	1,544				1,544
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, shares			165,031
Vesting of early exercised stock options	14				14
Stock-based compensation	4,782				4,782
Unrealized gains (losses), net of tax expense, (benefits)	70						70
Net loss	(18,413)							(18,413)
Ending balance at Jun. 30, 2018	387,033		$ 4		558,376		(259)	(171,088)
Ending balance, shares at Jun. 30, 2018			40,049,160
Beginning balance at Dec. 31, 2018	370,567		$ 4		573,183		(67)	(202,553)
Beginning balance, shares at Dec. 31, 2018			40,288,949
Issuance of common stock during offering, net of issuance costs, value		$ 271,213		$ 1		$ 271,212
Issuance of common stock during offering, net of issuance costs, shares				5,663,750
Issuance of common stock upon the exercise of options and release of stock awards	280				280
Issuance of common stock upon the exercise of options and release of stock awards, shares			49,076
Vesting of early exercised stock options	8				8
Stock-based compensation	6,981				6,981
Unrealized gains (losses), net of tax expense, (benefits)	363						363
Net loss	(37,470)							(37,470)
Ending balance at Mar. 31, 2019	611,942		$ 5		851,664		296	(240,023)
Ending balance, shares at Mar. 31, 2019			46,001,775
Beginning balance at Dec. 31, 2018	$ 370,567		$ 4		573,183		(67)	(202,553)
Beginning balance, shares at Dec. 31, 2018			40,288,949
Issuance of common stock upon the exercise of options and release of stock awards, shares	88,629
Net loss	$ (75,644)
Ending balance at Jun. 30, 2019	584,185		$ 5		861,880		497	(278,197)
Ending balance, shares at Jun. 30, 2019			46,098,059
Beginning balance at Mar. 31, 2019	611,942		$ 5		851,664		296	(240,023)
Beginning balance, shares at Mar. 31, 2019			46,001,775
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, value	1,571				1,571
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, shares			96,284
Vesting of early exercised stock options	7				7
Stock-based compensation	8,638				8,638
Unrealized gains (losses), net of tax expense, (benefits)	201						201
Net loss	(38,174)							(38,174)
Ending balance at Jun. 30, 2019	$ 584,185		$ 5		$ 861,880		$ 497	$ (278,197)
Ending balance, shares at Jun. 30, 2019			46,098,059

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Jun. 30, 2018
Common Stock | Follow-On-Offering
Issuance costs	$ 17,638	$ 12,233

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flow from operating activities:
Net loss	$ (75,644)	$ (36,233)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	15,619	8,413
Depreciation	944	723
Amortization of discount on investments	(656)	(53)
Income tax benefit of unrealized gains on short and long-term investments	(218)
Change in operating assets and liabilities:
Receivable from collaboration partner		1,013
Prepaid expenses and other current assets	822	(728)
Operating lease right-of-use assets	1,279
Other long-term assets	125	(58)
Accounts payable	281	698
Accrued liabilities	4,428	1,388
Prepayment from collaboration partner	(10,717)	4,683
Other long-term liabilities	(1,390)	(88)
Deferred revenue		(11,970)
Net cash used in operating activities	(65,127)	(32,212)
Cash flow from investing activities:
Purchases of investments	(41,593)	(39,595)
Sales of investments	4,000
Maturities of investments	29,000	12,000
Purchases of property and equipment	(1,693)	(2,356)
Net cash used in investing activities	(10,286)	(29,951)
Cash flow from financing activities:
Proceeds from issuance of common stock in follow-on offerings, net of issuance and financing costs	271,224	182,069
Proceeds from exercise of stock options and employee stock purchase plan	1,837	2,350
Net cash provided by financing activities	273,061	184,419
Net increase in cash, cash equivalents and restricted cash	197,648	122,256
Cash, cash equivalents and restricted cash, beginning of period	248,265	224,857
Cash, cash equivalents and restricted cash, end of period	445,913	347,113
Non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	15	32
Unpaid financing-related costs	11	206
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 17	$ 386

Organization

Organization	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization 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. The Company’s 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. MyoKardia’s most advanced program, mavacamten, is in four clinical trials, including a pivotal Phase 3 study for the treatment of obstructive hypertrophic cardiomyopathy. A second clinical-stage candidate, MYK-491, is in a Phase 2a multiple-ascending dose study in patients with stable systolic heart failure. The Company was incorporated on June 8, 2012 in Delaware and its corporate headquarters and operations are in South San Francisco, California. Liquidity The Company has incurred significant operating losses since inception and has an accumulated deficit of $278.2 million as of June 30, 2019. The Company has relied on its ability to fund its operations through private and public equity financings, and to a lesser extent, through a license and collaboration arrangement with a collaboration partner, Sanofi S.A. (“Sanofi”), via its subsidiary, Aventis Inc. As discussed further in Note 3, the collaboration agreement with Sanofi ended on December 31, 2018 and the Company no longer records revenues from Sanofi nor has it received reimbursements of research and development expenses after June 30, 2019. The Company has not yet received regulatory approval to commercialize or sell any product and does not have customers. Management expects operating losses and negative operating cash flows to continue for the foreseeable future. As the Company continues to incur losses, a transition to profitability is dependent upon the successful development, approval, and commercialization of the Company’s products and product candidates and the achievement of a level of revenues adequate to support its cost structure. The Company’s ultimate success depends on the outcome of its research and development activities. The Company anticipates the need to raise additional capital to fully implement its business plan and intends to raise such capital through the issuance of additional equity, debt and/or strategic alliances with partner companies. There is no assurance that such financing will be available on terms acceptable to the Company, if at all. On March 8, 2018, the Company filed a Registration Statement on Form S-3ASR (the “2018 Shelf Registration Statement”) covering the potential offering, issuance, and sale of an indeterminate amount of common stock, preferred stock, debt securities, warrants and/or units. In March 2019, the Company completed a follow-on offering under the 2018 Shelf Registration Statement in which the Company issued 5,663,750 shares of common stock at a price of $51.00 per share, including 738,750 shares sold directly to the underwriters upon exercise of their option to purchase up to 738,750 shares of the Company’s common stock within 30 days of the offering. During the six months ended June 30, 2019, the Company received proceeds totaling approximately $271.2 million from the offering, net of underwriting discounts and commissions and offering expenses. As the Company had $602.4 million of cash, cash equivalents and investments (short-term and long-term) and management believes that these amounts will be sufficient to meet the Company’s anticipated operating and capital expenditure requirements for the twelve months following the issuance date of this Form 10-Q.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	2. Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed consolidated financial statements are unaudited, include the Company’s accounts and those of its wholly-owned subsidiary and have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The condensed consolidated balance sheet at December 31, 2018, has been derived from the audited consolidated financial statements as of that date but does not include all of the information and footnotes required by GAAP for complete financial statements. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full fiscal year or any interim period and should be read in conjunction with the audited financial statements for the year ended December 31, 2018 and the notes thereto, which are included in the Company’s Annual Report on Form 10-K as of and for the year ended December 31, 2018. The significant accounting policies used in preparation of these condensed consolidated financial statements for the periods shown are consistent with those discussed in Note 2 to the consolidated financial statements in the Company’s 2018 Annual Report on Form 10-K and are updated below as necessary. The Company currently operates in one business segment, which is the identification, development and commercialization of therapies for the treatment of serious and neglected rare Accounting Policies Leases The Company determines if an arrangement is a lease at inception. Operating lease right-of-use (ROU) assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term beginning at the commencement date. As the Company’s leases do not provide enough information to determine an implicit interest rate, the Company determines an incremental borrowing rate based on the information available as of the lease commencement date in determining the present value of future payments. The operating lease ROU assets also include any lease payments made and excludes lease incentives and initial direct costs incurred. Lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Lease agreements that contain lease and non-lease components are accounted for as a single lease component. Restricted Cash A reconciliation of the Company’s cash, cash equivalents and restricted cash in the condensed consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows is as follows (in thousands): June 30, 2019 December 31, 2018 Cash and cash equivalents $ 443,693 $ 246,122 Restricted cash included in prepaid expenses and other current assets $ 364 — Restricted cash included in restricted cash and other 1,856 2,143 Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 445,913 $ 248,265 Use of Estimates The preparation of condensed consolidated financial statements in conformity with GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, other comprehensive gain (loss) and the related disclosures. On an ongoing basis, management evaluates its estimates, including estimates related to accrued clinical trial and manufacturing development expenses, stock-based compensation expense, income tax expense and operating leases. Significant estimates in these condensed consolidated financial statements include estimates made in connection with accrued research and development expenses, stock-based compensation expense, leases, income tax expenses and revenue. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions. Recently Adopted Accounting Pronouncements – Leases Effective January 1, 2019, the Company adopted Accounting Standards Codification Topic 842, Leases (ASC 842), which requires lessees to recognize a ROU asset and a lease liability on the balance sheet for all leases except for short-term leases with a lease term of twelve months or less. For lessees, leases continue to be classified as either operating or finance leases in the income statement. Lessor accounting is similar to the prior model but updated to align with certain changes to the lessee model. Lessors continue to classify leases as operating, direct financing or sales-type leases. The Company elected to adopt ASC 842 under the transition method that allows for the application of the new guidance at the beginning of the adoption period without recasting comparative periods. The Company also elected transition practical expedients to the implementation of the lease standard, as follows: (1) the Company did not reassess whether any expired or existing contracts, which had commenced before January 1, 2019, the date of adoption, are or contain leases, (2) the Company did not reassess the lease classification for any expired or existing leases and (3) the Company did not reassess the initial direct costs for any existing leases. Upon adoption, the Company recognized ROU assets and related lease liabilities totaling $2.1 million, representing the present value of future lease payments of each lease utilizing the Company’s incremental borrowing rate (“IBR”), which is the estimated borrowing rate of a collateralized loan over the remaining term of the lease. Also upon adoption, a deferred rent amount of $0.2 million as of December 31, 2018 was reclassified to the ROU assets, reducing the carrying value to $1.9 million. The increase in ROU assets and related lease liabilities since adoption of ASC 842 during the first quarter of 2019 resulted from the Company entering into an additional facility operating lease in South San Francisco. Recently Adopted Accounting Pronouncements - Other In June 2018, the FASB issued ASU No. 2018-07 (Topic 718), Compensation – Stock Compensation (ASU 2018-07) Recently Issued Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU 2018-13 (Topic 820), Fair Value Measurement (ASU 2018-13) In June 2016, the FASB issued ASU No. 2016-13 (Topic 326), Financial Instruments –Measurement of Credit Losses on Financial Instruments (ASU 2016-13),

Sanofi License and Collaboratio

Sanofi License and Collaboration Agreement	6 Months Ended
	Jun. 30, 2019
Collaboration And License Agreement Disclosure [Abstract]
Sanofi License and Collaboration Agreement	3. Sanofi License and Collaboration Agreement Sanofi (Aventis Inc.) Agreement Overview, Termination and Royalties In August 2014, the Company entered into an exclusive License and Collaboration Agreement (“Collaboration Agreement”) with Aventis Inc., a wholly-owned subsidiary of 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. During the period from August 2014 through December 2018, Sanofi paid the Company a total of $105.0 million in cash to perform research and development on the development of such products, as well as for granting to Sanofi certain royalty-bearing licenses. Of the $105.0 million, $0.7 million was attributed to a freestanding convertible preferred stock call option and $104.3 million was recognized as revenue during the period from August 2014 through December 31, 2018, the date on which the Company received a notice of termination of the Collaboration Agreement. In addition, in July 2019, the Company agreed to pay an aggregate of $80 million to Sanofi in consideration of Sanofi’s release of the Company from its royalty payment obligations on net sales of HCM-1 products set forth in the Collaboration Agreement; see Note 12 “Subsequent Events”. As a result, there are no further financial rights or obligations between the parties except for the final settlement of the Registration Program Plan (RPP); see “Cost Sharing” below. The Collaboration Agreement provided for a termination clause whereby on or before December 31, 2018, Sanofi was required to notify the Company of its intent to continue the collaboration. The continuation would have committed Sanofi to specific research and development activities in support of the commercialization of the Company’s products as well as resulted in a continuation of its obligation under the cost sharing portion of the collaboration to co-fund development as discussed further below. On December 31, 2018 , Sanofi notified the Company it was terminating the Collaboration Agreement. Under the terms of the termination: • Sanofi would reimburse the Company for certain research and development costs through June 30, 2019, after which such time such reimbursements were to be discontinued; • the Company recovered global rights to all programs in its portfolio, including lead clinical-stage candidates, mavacamten and MYK-491; and • Sanofi also would have remained eligible to receive royalties associated with any potential HCM-1 products that would have ranged from mid-single to low-double digits in the U.S. (there is no royalty obligation to Sanofi for sales outside the U.S.). However, as further discussed in Note 12, Subsequent Events, in July 2019 the Company was released from such royalty obligations upon its agreement to pay Sanofi an aggregate of $80 million, which resulted in the Company retaining exclusive worldwide rights to mavacamten and MYK-224. The Company determined that Sanofi was a related party of the Company due to its previous collaborative relationship and that it was the Company’s only partner. As of December 31, 2018, Sanofi was also a beneficial shareholder of the Company’s common stock. In February 2019, Sanofi sold all of its holdings of the Company’s common stock and no longer holds an equity interest in the Company. History of the Collaboration Agreement Under the Collaboration Agreement, the Company granted Sanofi royalty-bearing licenses to develop and commercialize products resulting from its lead candidate programs HCM-1, HCM-2 and DCM-1. The licenses provided Sanofi with worldwide rights in the case of DCM-1 and rights outside the United States with respect to the HCM-1 and HCM-2 programs. The terms of the Collaboration Agreement also stated that the Company was responsible for conducting research and development activities through early human efficacy studies for all three programs, except for specified research activities to be conducted by Sanofi. Upon entering into this agreement, the Company received an up-front non-refundable cash payment of $35.0 million and Sanofi made an up-front equity purchase of $10.0 million (additional equity investments from Sanofi totaling $26.5 million were received subsequent to the effective date of the Collaboration Agreement). The Company was also eligible to receive additional payments and services, as follows: • a one-time, non-refundable payment of $25.0 million contingent upon submission of an Investigational New Drug (“IND”) application before certain regulatory authorities for its DCM-1 program; • a non-refundable continuation payment of $45.0 million contingent upon Sanofi’s notification of its decision to continue the agreement beyond December 31, 2016; • up to $15.0 million in research and development funding for the lead compound in each program if studies leading to proof-of-concept (“POC”) were extended beyond December 31, 2018; and • up to $45.0 million in funding from Sanofi of approved in-kind research and clinical activities. During the fourth quarter of 2016, the Company submitted an IND application to the U.S. Food and Drug Administration and as a result, the Company received the $25.0 million milestone payment from Sanofi. In December 2016, Sanofi provided notice to the Company of its election to continue the collaboration through December 31, 2018 pursuant to the terms of the Collaboration Agreement. In connection with Sanofi’s decision to continue the collaboration, the Company received the $45.0 million milestone payment in January 2017. Under the terms of the agreement, the Company was also entitled to receive tiered royalties ranging from the mid-single digits to the mid-teens on net sales of certain HCM-1, HCM-2 and DCM-1 finished products outside the United States and on net sales of certain DCM-1 finished products in the United States. In July 2019, the Company and Sanofi entered into a Termination Agreement to clarify or amend certain rights and obligations of the parties surviving the Collboration Agreement. As further discussed in Note 12, Subsequent Events, in July 2019 the Company reacquired its royalty rights from Sanofi for $80 million, which resulted in the Company retaining exclusive worldwide rights to mavacamten and MYK-224. As a result of the repurchase of these rights, there are no further financial rights or obligations between the Company and Sanofi except for the final settlement of the RPP reimbursement arrangement. Revenue Recognition The Company evaluated the Collaboration Agreement and determined that it had the following promises: 1. the licenses of Company intellectual property to Sanofi for each of the HCM-1, HCM-2 and DCM-1 programs, and 2. the performance of research and development services, including regulatory support, for each of the three programs. The Company considered whether the licenses had standalone functionality and were capable of being distinct; however, given the fact that the research and development services were of such a specialized nature that could only be performed by the Company and Sanofi could not benefit from the intellectual property licenses without the Company’s performance, the Company determined that the intellectual property licenses were not distinct from the research and development services and thus the license and research and development services were combined as a single performance obligation for each of the three programs. The Company also determined that performance under each of the three programs is a separate performance obligation. Contract Term For revenue recognition purposes, the Company determined that the Collaboration Agreement was a period to period contract for which the Company had enforceable rights and obligations from inception through the initial term of December 31, 2016. Sanofi had the right to terminate the Collaboration Agreement prior to December 31, 2016 or to extend the contract term through December 31, 2018. If Sanofi had elected to terminate the agreement, the termination would have taken effect on December 31, 2016 and all licensed rights would have reverted to the Company. The Company did not have any obligation to reimburse Sanofi any portion of the payments received if Sanofi had terminated the agreement. In December 2016, Sanofi elected to continue the Collaboration Agreement through an extended term ending December 31, 2018 and made the $45.0 million continuation payment to the Company in January 2017. The Company determined that the extended term was to be treated as a separate contract because such an extension was not probable at the inception of the contract, the extension represented additional goods and services, and such activities were priced commensurate to the effort required and do not involve any significant discount. It was also concluded that the extended term provided the Company with enforceable rights and obligations for the two-year period ended December 31, 2018. Because Sanofi retained the option in the Collaboration Agreement to extend the arrangement, for purposes of revenue recognition neither party was committed to perform and the contract did not have enforceable rights and obligations which impacted revenue recognition beyond December 31, 2018. Transaction Price The Company’s assessment of the transaction price included an analysis of amounts to which it was expected to be entitled for providing goods or services to the customer which at contract inception consisted of the upfront cash payment, valued at $34.3 million, net of the fair value of $0.7 million allocated to the option provided to Sanofi to acquire equity, and variable consideration of $25.0 million, subject to an IND application. Sanofi paid the Company the $25.0 million milestone payment upon the Company’s application for the IND. In 2016, after the IND application was made and when the Company determined it was deemed probable that significant reversal in the amount of cumulative revenue recognized will not occur, the Company included this amount in the transaction price. As of December 31, 2016, all performance obligations associated with the initial term were satisfied. The extended term (from January 1, 2017 to December 31, 2018) had a fixed fee of $45.0 million, paid by Sanofi contemporaneously with the notice of continuation of the contract. The Company therefore determined that the transaction price for this extended term was $45.0 million. As previously noted above, the Collaboration Agreement also included up to $45.0 million in funding from Sanofi of approved in-kind research and clinical activities. Sanofi was the decision maker on how to provide these services and such services were used in the development of joint program technology which is co-owned by both parties. As such the Company concluded that these in-kind contributions did not constitute consideration paid by Sanofi to the Company. Any consideration related to sales-based royalties was to be recognized when the related sales occurred and therefore was also excluded from the transaction price. Methodology for Recognition Since the Company determined that the three performance obligations were satisfied over time, the Company selected a single revenue recognition method that it believed most faithfully depicted the Company’s performance in transferring control of the services. GAAP allows entities to choose between two methods to measure progress toward complete satisfaction of a performance obligation: 1. Output methods - recognize revenue on the basis of direct measurements of the value to the customer of the goods or services transferred to date relative to the remaining goods or services promised under the contract (e.g. surveys of performance completed to date, appraisals of results achieved, milestones reached, time elapsed, and units produced, or units delivered); or 2. Input methods - recognize revenue on the basis of the entity’s efforts or inputs to the satisfaction of a performance obligation (e.g., resources consumed, labor hours expended, costs incurred, or time elapsed) relative to the total expected inputs to the satisfaction of that performance obligation. The Company utilized a cost-based input method to measure proportional performance and calculated the corresponding amount of revenue to recognize. The Company believed this was the best measure of progress because other measures did not reflect how the Company executed its performance obligations under the contract with Sanofi. In applying the cost-based input methods of revenue recognition, the Company used actual costs incurred relative to budgeted costs to fulfill the combined performance obligations. Revenue was recognized based on actual costs incurred as a percentage of total actual and budgeted costs as the Company completed its performance obligations, which were fulfilled on December 31, 2018. A cost-based input method of revenue recognition requires management to make estimates of costs to complete the Company’s performance obligations. In making such estimates, significant judgment is required to evaluate assumptions related to cost estimates. The cumulative effect of revisions to estimated costs to complete the Company’s performance obligations were recorded in the period in which changes were identified and amounts could be reasonably estimated. For the six months ended June 30, 2019 and 2018, the Company recognized zero and $12.0 million of collaboration and license revenue, respectively. The Company will not recognize any further revenue from the Collaboration Agreement. There were no contract assets or liabilities as of June 30, 2019. The following table presents changes in the Company’s contract liabilities, which excludes research and development reimbursements under the cost sharing plan further discussed below, for the six months ended June 30, 2018 (in thousands): Six Months Ended June 30, 2018 Balance at Beginning of Period Additions Deductions Balance at End of Period Contract liabilities: Deferred revenue $ 33,558 $ — $ (11,970 ) $ 21,588 Cost Sharing During the six months ended June 30, 2019 and 2018, the Company received research and development cost reimbursements from Sanofi under the terms of the Collaboration Agreement. Since the inception of the Collaboration Agreement and up until the termination date: (i) Sanofi had been conditionally responsible for reimbursing the Company for one half or more of the RPP costs after clinical proof-of-concept had been established for the lead compound under each of the HCM-1 and HCM-2 programs; and (ii) if the Company had initiated a clinical trial of a compound under a proof-of-concept development plan and not terminated its development thereof and if another additional compound had been identified as a development candidate for the same program, the Company was entitled to full reimbursement of pre-proof-of-concept (“pre-POC”) research and development costs on development candidates mutually identified as such additional compounds, with the objective of conducting IND-enabling studies and clinical trials on such candidate. Effective October 2017 and until June 30, 2019 , Sanofi shared RPP costs for the mavacamten program pursuant to the Collaboration Agreement termination terms. RPP costs approved by the Company and Sanofi included amounts incurred relating to clinical trials, development and manufacturing of, and obtaining regulatory approvals for mavacamten, and include d direct employee costs and direct out-of-pocket costs incurred, by or on behalf of a party, specifically identifiable or reasonably and directly allocable to those activities. Pursuant to the additional compounds provisions of the Collaboration Agreement, in August 2018 Sanofi agreed to reimburse the Company for eligible costs it has incurred in the development of the MYK-224 compound, which was identified as an additional compound under the HCM-1 program. Eligible costs were subject to review and approval under the same procedures as under the RPP program; reimbursable costs consisted of research and development activities agreed to by the Company and Sanofi that were negotiated and budgeted prior to the application for reimbursement. Reimbursements for this compound were received from Sanofi until June 30, 2019, in accordance with the Collaboration Agreement termination terms. Estimated reimbursements were invoiced to Sanofi before each interim period based on budgeted amounts. For the RPP program, these estimates consisted of one half of the Company’s mavacamten development budget in excess of Sanofi’s mavacamten development budget each interim period. For the MYK-224 compound, these estimates consisted of the Company’s entire research and development budget related to the compound for the forthcoming quarter. After each period end, a review of the actual expenses incurred was performed and any adjustments were carried forward to future invoices. Actual amounts received from Sanofi were applied to the applicable interim period to reduce the Company’s research and development expenses. Due to the termination of the license agreement effective December 31, 2018, the Company will not receive further reimbursements for the MYK-224 compound for any periods subsequent to April 1, 2019 and will not participate in the cost sharing arrangement for the mavacamten compound after June 30, 2019. As a result, there are no further financial rights or obligations between the Company and Sanofi whether for revenue recognition purposes or for research and development cost sharing except for the final settlement of the cost sharing arrangement, which is anticipated to be finalized in the third quarter of 2019. The Company recorded $18.5 The following table presents the Sanofi research and development reimbursement receivables and related prepayment activity during the six months ended June 30, 2019 and 2018 (in thousands): Six Months Ended June 30, 2019 2018 Receivable from collaboration partner Balance at beginning of period $ — $ 1,013 Deductions — (1,013 ) Balance at end of period $ — $ — Prepayment from collaboration partner for mavacamten Balance at beginning of period $ 12,973 $ 4,432 Payments received from Sanofi 7,777 11,809 Actual expenses incurred (18,494 ) (7,126 ) Balance at end of period $ 2,256 $ 9,115

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. 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 June 30, 2019 Total Level 1 Level 2 Level 3 Assets Money market funds $ 443,632 $ 443,632 $ — $ — U.S. government agency obligations 98,622 — 98,622 — Corporate securities 60,122 — 60,122 — Total $ 602,376 $ 443,632 $ 158,744 $ — Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 Assets Money market funds $ 245,194 $ 245,194 $ — $ — U.S. government agency obligations 85,033 — 85,033 — Corporate securities 63,679 — 63,679 — Total $ 393,906 $ 245,194 $ 148,712 $ — The following table is a summary of amortized cost, unrealized gain and loss, and fair value of the Company’s marketable securities by contractual maturities (in thousands): Fair Value Measurements at June 30, 2019 Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 443,632 $ — $ — $ 443,632 Short-term investments (due within one year) 114,486 306 — 114,792 Long-term investments (due between one and two years) 43,537 415 — 43,952 Total $ 601,655 $ 721 $ — $ 602,376 Fair Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 245,194 $ — $ — $ 245,194 Short-term investments (due within one year) 68,656 — (92 ) 68,564 Long-term investments (due between one and two years) 80,118 98 (68 ) 80,148 Total $ 393,968 $ 98 $ (160 ) $ 393,906

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Lessee Disclosure [Abstract]
Leases	5. Leases As of June 30, 2019, t he Company has operating leases recorded on its balance sheet for approximately 70,500 square feet of office and lab space in three separate facilities in South San Francisco, California (the “Existing Facilities”). All of the lease agreements associated with the Existing Facilities expire on or before April 2020 and there are no options to extend the leases. The Company does not plan to cancel the existing lease agreements for its Existing Facilities prior to their respective expiration dates. The Company does not have any leases that would be classified as finance leases. Information related to operating leases as of June 30, 2019 and upon adoption of ASC 842 on January 1, 2019 is as follows (in thousands, except for percentages and years). June 30, 2019 January 1, 2019 Assets Operating lease right-of-use assets $ 1,756 $ 1,940 Liabilities Operating lease liabilities - current $ 1,831 $ 2,126 Operating lease liabilities - noncurrent — — $ 1,831 $ 2,126 Weighted average remaining lease term (years) 0.7 1.0 Weighted average discount rate 6 % 6 % Information related to operating lease activity during the six months ended June 30, 2019 follows (in thousands): Six Months Ended June 30, 2019 Operating lease right-of-use assets obtained in exchange for lease obligations $ 1,095 Operating lease rental expense $ 1,415 Operating lease payments $ 1,528 Future annual payments of operating lease liabilities as of June 30, 2019 are as follows (in thousands): Year ending December 31: Amount 2019 (six months remaining) $ 1,478 2020 387 Total future lease payments 1,865 Less: imputed interest (34 ) Total operating lease liabilities $ 1,831 In September 2018, the Company entered into a noncancelable operating lease (the “Lease”) for approximately 129,800 square feet of space in Brisbane, California (the “New Facility”). The date on which the Company will record the ROU asset and lease liability on its balance sheet (the “Commencement Date”), as well as when it becomes responsible for paying rent under the Lease, will be the date the premises are ready for occupancy, currently anticipated to be January 2020. The Lease expires 10 years after the Commencement Date. The Lease grants the Company an option to extend the Lease for an additional 10-year period. Future minimum rental payments under the Lease during the 10-year term are $93.2 million in the aggregate. The Lease further provides that the Company is obligated to pay to the landlord certain costs, including taxes and operating expenses. In September 2018, t he Company provided a standby letter of credit of $1.9 million as security for its obligations under the Lease. This standby letter of credit, together with standby letters on Existing Facilities, are included on the balance sheet in prepaid expenses and other current assets and in restricted cash and other. Future annual minimum operating lease payments due under the Lease for the New Facility are as follows (in thousands): Year ending December 31: Amount 1 2019 (six months remaining) $ — 2020 5,454 2021 8,461 2022 8,757 2023 9,063 Thereafter 61,444 Total $ 93,179 (1) The table above is prepared under the assumption that the Commencement Date at the New Facility will be January 1, 2020. Future annual minimum lease payments for operating leases as of December 31, 2018 were as follows (in thousands): Year ending December 31: Amount 2019 2,752 2020 5,831 2021 8,461 2022 8,757 2023 9,063 Thereafter 61,444 Total 96,308 The adoption of ASC 842 did not materially affect the amount or timing of operating lease rent expense to be recognized during the year ended December 31, 2019 as compared to accounting under the prior guidance. Operating lease r ent expense, which is included in operating expenses on the Company’s condensed consolidated statements of operations and comprehensive loss, was $0.7 million and $0.6 million for the three months ended June 30, 2019 and 2018, respectively, and $1.4 million and $1.0 million for the six months ended June 30, 2019 and 2018, respectively. The operating leases require the Company to share in prorated operating expenses and property taxes based upon actual amounts incurred; those amounts are not fixed for future periods and, therefore, are not included in the future commitments listed above

Balance Sheet Components

Balance Sheet Components	6 Months Ended
	Jun. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	6. Balance Sheet Components Property and Equipment Property and equipment consist of the following (in thousands): June 30, 2019 December 31, 2018 Scientific equipment $ 9,944 $ 9,126 Furniture and equipment 1,471 1,248 Capitalized software 389 302 Leasehold improvements 564 451 Total 12,368 11,127 Less: Accumulated depreciation (6,933 ) (5,989 ) Property and equipment, net $ 5,435 $ 5,138 Depreciation expense was $0.4 million and $0.4 million for the three months ended June 30, 2019 and 2018, respectively, and $0.9 million and $0.7 million for the six months ended June 30, 2019 and 2018, respectively. Accrued Liabilities Accrued liabilities consist of the following (in thousands): June 30, 2019 December 31, 2018 Clinical research and development $ 16,568 $ 10,903 Payroll-related liabilities 6,919 8,151 Other 1,257 1,704 Total accrued liabilities $ 24,744 $ 20,758

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. 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. 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 June 30, 2019 or December 31, 2018. 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	6 Months Ended
	Jun. 30, 2019
Stockholders Equity Note [Abstract]
Stockholders' Equity	8. Stockholders’ Equity On March 8, 2018, the Company filed the 2018 Shelf Registration Statement covering the potential offering, issuance, and sale of an indeterminate amount of shares of common stock at a price of $51.00 per share, which included 738,750 shares sold directly to the underwriters upon exercise of their over-allotment option. During the six months ended June 30, 2019, the Company received proceeds totaling Common Stock Reserved for Issuance The Company has reserved shares of common stock for issuance as follows: June 30, 2019 December 31, 2018 Options and awards issued and outstanding 5,303,268 3,864,407 Shares available for issuance under 2015 Stock Option and Incentive Plan 956,329 904,785 Shares available for issuance under 2015 Employee Stock Purchase Plan 1,162,647 780,716 Total 7,422,244 5,549,908

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9. 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 equity awards granted to employees and consultants for all periods presented (in thousands): Three Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 3,795 $ 2,050 $ 6,758 $ 3,641 Selling, general and administrative 4,843 2,732 8,861 4,772 Total $ 8,638 $ 4,782 $ 15,619 $ 8,413 The following summarizes option and other equity award 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, 2018 3,701,461 $ 26.40 Options granted 1,179,253 41.34 Options exercised (88,629 ) 11.23 Options canceled/forfeited (71,235 ) 32.24 Balance at June 30, 2019 4,720,850 30.33 Shares Subject to Weighted Average Outstanding Awards Grant Date Fair Value Balance at December 31, 2018 162,946 $ 53.37 RSUs awarded 460,149 41.54 RSUs released (32,523 ) 52.06 RSUs forfeited (8,154 ) 47.25 Balance at June 30, 2019 582,418 44.19 Restricted stock units (“RSUs”) settle into shares of common stock upon vesting and the fair value is the market price on the date of grant. Pursuant to the terms of the Company’s 2015 Employee Stock Purchase Plan (the “2015 ESPP”), on April 30, 2019, the Company issued 20,958 shares to participants in the 2015 ESPP in exchange for their contributions during the period from November 1, 2018 to April 30, 2019. In relation to stock options and awards that vest upon the achievement of performance criteria, there was $0 and $40,000 in stock-based compensation expense recorded for the three months ended June 30, 2019 and 2018, respectively, and was recorded for the six months ended June 30, 2019 and 2018, respectively.

Net Loss per Share

Net Loss per Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Share	10. 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 June 30, Six Months Ended June 30, 2019 2018 2019 2018 Numerator Net loss $ (38,174 ) $ (18,413 ) $ (75,644 ) $ (36,233 ) Denominator Weighted average shares outstanding 46,067,571 37,490,960 43,305,587 36,694,820 Less: weighted average shares subject to repurchase (1,670 ) (50,936 ) (4,170 ) (74,073 ) Weighted average shares used to compute basic and diluted net loss per share 46,065,901 37,440,024 43,301,417 36,620,747 Net loss per share, basic and diluted $ (0.83 ) $ (0.49 ) $ (1.75 ) $ (0.99 ) 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 June 30, 2019 2018 Common stock subject to repurchase — 32,361 Options and awards issued and outstanding 5,303,268 3,904,668 As of June 30, 2019, the Company has contributions from plan participants of $0.3 million under the 2015 ESPP, which if converted, would be equivalent to approximately 8,000 shares based on 85% of the stock price at the beginning of the offering period. As of June 30, 2018, the Company had contributions from plan participants of $0.2 million under the 2015 ESPP, which if converted, would have been equivalent to approximately 5,000 shares based on 85% of the stock price at the beginning of the offering period.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	11. Income taxes Deferred tax assets and deferred tax liabilities are determined based on temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some portion or all of the deferred tax assets will not be realized. The Company does not recognize a tax benefit for uncertain tax positions unless it is more likely than not that the position will be sustained upon examination by tax authorities, including resolution of any related appeals or litigation processes, based on the technical merits of the position. The tax benefit potentially recorded for these positions is measured at the largest amount of cumulative benefit that has greater than a 50 percent likelihood of being realized upon ultimate settlement. Deferred tax assets that do not meet these recognition criteria are not recorded and the Company recognizes a liability for uncertain tax positions that may result in tax payments. If such unrecognized tax benefits were realized and not subject to valuation allowances, the entire amount would impact the tax provision. As of June 30, 2019, the Company ’ The Company recognized a benefit from income taxes of $218,000 for the three and six months ended June 30, 2019 and $0 for the three and six months ended June 30, 2018. The income tax benefit is primarily related to the net unrealized gain on the Company's short term and long-term investments, resulting in an increase in deferred tax liabilities and a decrease in the valuation allowance.

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events On July 18, 2019, the Company announced an agreement with Sanofi to reacquire the U.S. royalty rights to mavacamten and MYK-224 for $80 million, of which $50 million was paid immediately and $30 million was transferred to escrow to be paid by June 30, 2020. The agreement also required the Company to pay $4.3 million to Sanofi for certain of its assets related to the MYK-491 program. All amounts were paid in July 2019. Neither the Company nor Sanofi have further obligations under the agreement that would prevent the payment of the escrowed amount. As a result of the agreement, there are no further financial rights or obligations between the Company and Sanofi except for the final settlement of the cost sharing arrangement described in Note 3, which will occur in the third quarter of 2019.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements are unaudited, include the Company’s accounts and those of its wholly-owned subsidiary and have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The condensed consolidated balance sheet at December 31, 2018, has been derived from the audited consolidated financial statements as of that date but does not include all of the information and footnotes required by GAAP for complete financial statements. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full fiscal year or any interim period and should be read in conjunction with the audited financial statements for the year ended December 31, 2018 and the notes thereto, which are included in the Company’s Annual Report on Form 10-K as of and for the year ended December 31, 2018. The significant accounting policies used in preparation of these condensed consolidated financial statements for the periods shown are consistent with those discussed in Note 2 to the consolidated financial statements in the Company’s 2018 Annual Report on Form 10-K and are updated below as necessary. The Company currently operates in one business segment, which is the identification, development and commercialization of therapies for the treatment of serious and neglected rare
Accounting Policies - Leases	Accounting Policies Leases The Company determines if an arrangement is a lease at inception. Operating lease right-of-use (ROU) assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term beginning at the commencement date. As the Company’s leases do not provide enough information to determine an implicit interest rate, the Company determines an incremental borrowing rate based on the information available as of the lease commencement date in determining the present value of future payments. The operating lease ROU assets also include any lease payments made and excludes lease incentives and initial direct costs incurred. Lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Lease agreements that contain lease and non-lease components are accounted for as a single lease component.
Restricted Cash	Restricted Cash A reconciliation of the Company’s cash, cash equivalents and restricted cash in the condensed consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows is as follows (in thousands): June 30, 2019 December 31, 2018 Cash and cash equivalents $ 443,693 $ 246,122 Restricted cash included in prepaid expenses and other current assets $ 364 — Restricted cash included in restricted cash and other 1,856 2,143 Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 445,913 $ 248,265
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in conformity with GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, other comprehensive gain (loss) and the related disclosures. On an ongoing basis, management evaluates its estimates, including estimates related to accrued clinical trial and manufacturing development expenses, stock-based compensation expense, income tax expense and operating leases. Significant estimates in these condensed consolidated financial statements include estimates made in connection with accrued research and development expenses, stock-based compensation expense, leases, income tax expenses and revenue. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements – Leases Effective January 1, 2019, the Company adopted Accounting Standards Codification Topic 842, Leases (ASC 842), which requires lessees to recognize a ROU asset and a lease liability on the balance sheet for all leases except for short-term leases with a lease term of twelve months or less. For lessees, leases continue to be classified as either operating or finance leases in the income statement. Lessor accounting is similar to the prior model but updated to align with certain changes to the lessee model. Lessors continue to classify leases as operating, direct financing or sales-type leases. The Company elected to adopt ASC 842 under the transition method that allows for the application of the new guidance at the beginning of the adoption period without recasting comparative periods. The Company also elected transition practical expedients to the implementation of the lease standard, as follows: (1) the Company did not reassess whether any expired or existing contracts, which had commenced before January 1, 2019, the date of adoption, are or contain leases, (2) the Company did not reassess the lease classification for any expired or existing leases and (3) the Company did not reassess the initial direct costs for any existing leases. Upon adoption, the Company recognized ROU assets and related lease liabilities totaling $2.1 million, representing the present value of future lease payments of each lease utilizing the Company’s incremental borrowing rate (“IBR”), which is the estimated borrowing rate of a collateralized loan over the remaining term of the lease. Also upon adoption, a deferred rent amount of $0.2 million as of December 31, 2018 was reclassified to the ROU assets, reducing the carrying value to $1.9 million. The increase in ROU assets and related lease liabilities since adoption of ASC 842 during the first quarter of 2019 resulted from the Company entering into an additional facility operating lease in South San Francisco. Recently Adopted Accounting Pronouncements - Other In June 2018, the FASB issued ASU No. 2018-07 (Topic 718), Compensation – Stock Compensation (ASU 2018-07)
Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Issued Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU 2018-13 (Topic 820), Fair Value Measurement (ASU 2018-13) In June 2016, the FASB issued ASU No. 2016-13 (Topic 326), Financial Instruments –Measurement of Credit Losses on Financial Instruments (ASU 2016-13),
Revenue Recognition	Revenue Recognition The Company evaluated the Collaboration Agreement and determined that it had the following promises: 1. the licenses of Company intellectual property to Sanofi for each of the HCM-1, HCM-2 and DCM-1 programs, and 2. the performance of research and development services, including regulatory support, for each of the three programs. The Company considered whether the licenses had standalone functionality and were capable of being distinct; however, given the fact that the research and development services were of such a specialized nature that could only be performed by the Company and Sanofi could not benefit from the intellectual property licenses without the Company’s performance, the Company determined that the intellectual property licenses were not distinct from the research and development services and thus the license and research and development services were combined as a single performance obligation for each of the three programs. The Company also determined that performance under each of the three programs is a separate performance obligation. Contract Term For revenue recognition purposes, the Company determined that the Collaboration Agreement was a period to period contract for which the Company had enforceable rights and obligations from inception through the initial term of December 31, 2016. Sanofi had the right to terminate the Collaboration Agreement prior to December 31, 2016 or to extend the contract term through December 31, 2018. If Sanofi had elected to terminate the agreement, the termination would have taken effect on December 31, 2016 and all licensed rights would have reverted to the Company. The Company did not have any obligation to reimburse Sanofi any portion of the payments received if Sanofi had terminated the agreement. In December 2016, Sanofi elected to continue the Collaboration Agreement through an extended term ending December 31, 2018 and made the $45.0 million continuation payment to the Company in January 2017. The Company determined that the extended term was to be treated as a separate contract because such an extension was not probable at the inception of the contract, the extension represented additional goods and services, and such activities were priced commensurate to the effort required and do not involve any significant discount. It was also concluded that the extended term provided the Company with enforceable rights and obligations for the two-year period ended December 31, 2018. Because Sanofi retained the option in the Collaboration Agreement to extend the arrangement, for purposes of revenue recognition neither party was committed to perform and the contract did not have enforceable rights and obligations which impacted revenue recognition beyond December 31, 2018. Transaction Price The Company’s assessment of the transaction price included an analysis of amounts to which it was expected to be entitled for providing goods or services to the customer which at contract inception consisted of the upfront cash payment, valued at $34.3 million, net of the fair value of $0.7 million allocated to the option provided to Sanofi to acquire equity, and variable consideration of $25.0 million, subject to an IND application. Sanofi paid the Company the $25.0 million milestone payment upon the Company’s application for the IND. In 2016, after the IND application was made and when the Company determined it was deemed probable that significant reversal in the amount of cumulative revenue recognized will not occur, the Company included this amount in the transaction price. As of December 31, 2016, all performance obligations associated with the initial term were satisfied. The extended term (from January 1, 2017 to December 31, 2018) had a fixed fee of $45.0 million, paid by Sanofi contemporaneously with the notice of continuation of the contract. The Company therefore determined that the transaction price for this extended term was $45.0 million. As previously noted above, the Collaboration Agreement also included up to $45.0 million in funding from Sanofi of approved in-kind research and clinical activities. Sanofi was the decision maker on how to provide these services and such services were used in the development of joint program technology which is co-owned by both parties. As such the Company concluded that these in-kind contributions did not constitute consideration paid by Sanofi to the Company. Any consideration related to sales-based royalties was to be recognized when the related sales occurred and therefore was also excluded from the transaction price. Methodology for Recognition Since the Company determined that the three performance obligations were satisfied over time, the Company selected a single revenue recognition method that it believed most faithfully depicted the Company’s performance in transferring control of the services. GAAP allows entities to choose between two methods to measure progress toward complete satisfaction of a performance obligation: 1. Output methods - recognize revenue on the basis of direct measurements of the value to the customer of the goods or services transferred to date relative to the remaining goods or services promised under the contract (e.g. surveys of performance completed to date, appraisals of results achieved, milestones reached, time elapsed, and units produced, or units delivered); or 2. Input methods - recognize revenue on the basis of the entity’s efforts or inputs to the satisfaction of a performance obligation (e.g., resources consumed, labor hours expended, costs incurred, or time elapsed) relative to the total expected inputs to the satisfaction of that performance obligation. The Company utilized a cost-based input method to measure proportional performance and calculated the corresponding amount of revenue to recognize. The Company believed this was the best measure of progress because other measures did not reflect how the Company executed its performance obligations under the contract with Sanofi. In applying the cost-based input methods of revenue recognition, the Company used actual costs incurred relative to budgeted costs to fulfill the combined performance obligations. Revenue was recognized based on actual costs incurred as a percentage of total actual and budgeted costs as the Company completed its performance obligations, which were fulfilled on December 31, 2018. A cost-based input method of revenue recognition requires management to make estimates of costs to complete the Company’s performance obligations. In making such estimates, significant judgment is required to evaluate assumptions related to cost estimates. The cumulative effect of revisions to estimated costs to complete the Company’s performance obligations were recorded in the period in which changes were identified and amounts could be reasonably estimated.
Registration Program Plan and Pre-POC Cost Sharing	Cost Sharing During the six months ended June 30, 2019 and 2018, the Company received research and development cost reimbursements from Sanofi under the terms of the Collaboration Agreement. Since the inception of the Collaboration Agreement and up until the termination date: (i) Sanofi had been conditionally responsible for reimbursing the Company for one half or more of the RPP costs after clinical proof-of-concept had been established for the lead compound under each of the HCM-1 and HCM-2 programs; and (ii) if the Company had initiated a clinical trial of a compound under a proof-of-concept development plan and not terminated its development thereof and if another additional compound had been identified as a development candidate for the same program, the Company was entitled to full reimbursement of pre-proof-of-concept (“pre-POC”) research and development costs on development candidates mutually identified as such additional compounds, with the objective of conducting IND-enabling studies and clinical trials on such candidate. Effective October 2017 and until June 30, 2019 , Sanofi shared RPP costs for the mavacamten program pursuant to the Collaboration Agreement termination terms. RPP costs approved by the Company and Sanofi included amounts incurred relating to clinical trials, development and manufacturing of, and obtaining regulatory approvals for mavacamten, and include d direct employee costs and direct out-of-pocket costs incurred, by or on behalf of a party, specifically identifiable or reasonably and directly allocable to those activities. Pursuant to the additional compounds provisions of the Collaboration Agreement, in August 2018 Sanofi agreed to reimburse the Company for eligible costs it has incurred in the development of the MYK-224 compound, which was identified as an additional compound under the HCM-1 program. Eligible costs were subject to review and approval under the same procedures as under the RPP program; reimbursable costs consisted of research and development activities agreed to by the Company and Sanofi that were negotiated and budgeted prior to the application for reimbursement. Reimbursements for this compound were received from Sanofi until June 30, 2019, in accordance with the Collaboration Agreement termination terms. Estimated reimbursements were invoiced to Sanofi before each interim period based on budgeted amounts. For the RPP program, these estimates consisted of one half of the Company’s mavacamten development budget in excess of Sanofi’s mavacamten development budget each interim period. For the MYK-224 compound, these estimates consisted of the Company’s entire research and development budget related to the compound for the forthcoming quarter. After each period end, a review of the actual expenses incurred was performed and any adjustments were carried forward to future invoices. Actual amounts received from Sanofi were applied to the applicable interim period to reduce the Company’s research and development expenses. Due to the termination of the license agreement effective December 31, 2018, the Company will not receive further reimbursements for the MYK-224 compound for any periods subsequent to April 1, 2019 and will not participate in the cost sharing arrangement for the mavacamten compound after June 30, 2019. As a result, there are no further financial rights or obligations between the Company and Sanofi whether for revenue recognition purposes or for research and development cost sharing except for the final settlement of the cost sharing arrangement, which is anticipated to be finalized in the third quarter of 2019.
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. There were no contingent liabilities requiring accrual or disclosure as of June 30, 2019 or December 31, 2018.
Guarantees and Indemnifications	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.
Fair Value Measurements	4. 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.
Stock-Based Compensation	Stock-Based Compensation In relation to stock options and awards that vest upon the achievement of performance criteria, there was $0 and $40,000 in stock-based compensation expense recorded for the three months ended June 30, 2019 and 2018, respectively, and was recorded for the six months ended June 30, 2019 and 2018, respectively.

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Cash Reported in Condensed Consolidated Statements of Cash Flows	A reconciliation of the Company’s cash, cash equivalents and restricted cash in the condensed consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows is as follows (in thousands): June 30, 2019 December 31, 2018 Cash and cash equivalents $ 443,693 $ 246,122 Restricted cash included in prepaid expenses and other current assets $ 364 — Restricted cash included in restricted cash and other 1,856 2,143 Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 445,913 $ 248,265

Sanofi License and Collaborat_2

Sanofi License and Collaboration Agreement (Tables)	6 Months Ended
	Jun. 30, 2019
Collaboration And License Agreement Disclosure [Abstract]
Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities, which excludes research and development reimbursements under the cost sharing plan further discussed below, for the six months ended June 30, 2018 (in thousands): Six Months Ended June 30, 2018 Balance at Beginning of Period Additions Deductions Balance at End of Period Contract liabilities: Deferred revenue $ 33,558 $ — $ (11,970 ) $ 21,588
Schedule of Research and Development Reimbursement Receivables and Related Prepayments from Collaboration Partner	The following table presents the Sanofi research and development reimbursement receivables and related prepayment activity during the six months ended June 30, 2019 and 2018 (in thousands): Six Months Ended June 30, 2019 2018 Receivable from collaboration partner Balance at beginning of period $ — $ 1,013 Deductions — (1,013 ) Balance at end of period $ — $ — Prepayment from collaboration partner for mavacamten Balance at beginning of period $ 12,973 $ 4,432 Payments received from Sanofi 7,777 11,809 Actual expenses incurred (18,494 ) (7,126 ) Balance at end of period $ 2,256 $ 9,115

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Fair Value Assets 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 June 30, 2019 Total Level 1 Level 2 Level 3 Assets Money market funds $ 443,632 $ 443,632 $ — $ — U.S. government agency obligations 98,622 — 98,622 — Corporate securities 60,122 — 60,122 — Total $ 602,376 $ 443,632 $ 158,744 $ — Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 Assets Money market funds $ 245,194 $ 245,194 $ — $ — U.S. government agency obligations 85,033 — 85,033 — Corporate securities 63,679 — 63,679 — Total $ 393,906 $ 245,194 $ 148,712 $ —
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 of the Company’s marketable securities by contractual maturities (in thousands): Fair Value Measurements at June 30, 2019 Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 443,632 $ — $ — $ 443,632 Short-term investments (due within one year) 114,486 306 — 114,792 Long-term investments (due between one and two years) 43,537 415 — 43,952 Total $ 601,655 $ 721 $ — $ 602,376 Fair Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 245,194 $ — $ — $ 245,194 Short-term investments (due within one year) 68,656 — (92 ) 68,564 Long-term investments (due between one and two years) 80,118 98 (68 ) 80,148 Total $ 393,968 $ 98 $ (160 ) $ 393,906

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Lessee Disclosure [Abstract]
Schedule of Information Related to Operating Leases	Information related to operating leases as of June 30, 2019 and upon adoption of ASC 842 on January 1, 2019 is as follows (in thousands, except for percentages and years). June 30, 2019 January 1, 2019 Assets Operating lease right-of-use assets $ 1,756 $ 1,940 Liabilities Operating lease liabilities - current $ 1,831 $ 2,126 Operating lease liabilities - noncurrent — — $ 1,831 $ 2,126 Weighted average remaining lease term (years) 0.7 1.0 Weighted average discount rate 6 % 6 %
Schedule of Information Related to Operating Lease Activity	Information related to operating lease activity during the six months ended June 30, 2019 follows (in thousands): Six Months Ended June 30, 2019 Operating lease right-of-use assets obtained in exchange for lease obligations $ 1,095 Operating lease rental expense $ 1,415 Operating lease payments $ 1,528
Schedule of Future Annual Payment of Operating Lease Liabilities	Future annual payments of operating lease liabilities as of June 30, 2019 are as follows (in thousands): Year ending December 31: Amount 2019 (six months remaining) $ 1,478 2020 387 Total future lease payments 1,865 Less: imputed interest (34 ) Total operating lease liabilities $ 1,831 Future annual minimum operating lease payments due under the Lease for the New Facility are as follows (in thousands): Year ending December 31: Amount 1 2019 (six months remaining) $ — 2020 5,454 2021 8,461 2022 8,757 2023 9,063 Thereafter 61,444 Total $ 93,179 (1) The table above is prepared under the assumption that the Commencement Date at the New Facility will be January 1, 2020.
Schedule of Future Annual Minimum Operating Lease Payments Due Under Leases	Future annual minimum lease payments for operating leases as of December 31, 2018 were as follows (in thousands): Year ending December 31: Amount 2019 2,752 2020 5,831 2021 8,461 2022 8,757 2023 9,063 Thereafter 61,444 Total 96,308

Balance Sheet Components (Table

Balance Sheet Components (Tables)	6 Months Ended
	Jun. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): June 30, 2019 December 31, 2018 Scientific equipment $ 9,944 $ 9,126 Furniture and equipment 1,471 1,248 Capitalized software 389 302 Leasehold improvements 564 451 Total 12,368 11,127 Less: Accumulated depreciation (6,933 ) (5,989 ) Property and equipment, net $ 5,435 $ 5,138
Summary of Accrued Liabilities	Accrued liabilities consist of the following (in thousands): June 30, 2019 December 31, 2018 Clinical research and development $ 16,568 $ 10,903 Payroll-related liabilities 6,919 8,151 Other 1,257 1,704 Total accrued liabilities $ 24,744 $ 20,758

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance	The Company has reserved shares of common stock for issuance as follows: June 30, 2019 December 31, 2018 Options and awards issued and outstanding 5,303,268 3,864,407 Shares available for issuance under 2015 Stock Option and Incentive Plan 956,329 904,785 Shares available for issuance under 2015 Employee Stock Purchase Plan 1,162,647 780,716 Total 7,422,244 5,549,908

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense Related to Equity Awards Granted to Employees and Consultants	The following table sets forth stock-based compensation expense related to equity awards granted to employees and consultants for all periods presented (in thousands): Three Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 3,795 $ 2,050 $ 6,758 $ 3,641 Selling, general and administrative 4,843 2,732 8,861 4,772 Total $ 8,638 $ 4,782 $ 15,619 $ 8,413
Summary of Stock Option and Other Equity Award Activity Under Plans	The following summarizes option and other equity award 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, 2018 3,701,461 $ 26.40 Options granted 1,179,253 41.34 Options exercised (88,629 ) 11.23 Options canceled/forfeited (71,235 ) 32.24 Balance at June 30, 2019 4,720,850 30.33 Shares Subject to Weighted Average Outstanding Awards Grant Date Fair Value Balance at December 31, 2018 162,946 $ 53.37 RSUs awarded 460,149 41.54 RSUs released (32,523 ) 52.06 RSUs forfeited (8,154 ) 47.25 Balance at June 30, 2019 582,418 44.19

Net Loss per Share (Tables)

Net Loss per Share (Tables)	6 Months Ended
	Jun. 30, 2019
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 June 30, Six Months Ended June 30, 2019 2018 2019 2018 Numerator Net loss $ (38,174 ) $ (18,413 ) $ (75,644 ) $ (36,233 ) Denominator Weighted average shares outstanding 46,067,571 37,490,960 43,305,587 36,694,820 Less: weighted average shares subject to repurchase (1,670 ) (50,936 ) (4,170 ) (74,073 ) Weighted average shares used to compute basic and diluted net loss per share 46,065,901 37,440,024 43,301,417 36,620,747 Net loss per share, basic and diluted $ (0.83 ) $ (0.49 ) $ (1.75 ) $ (0.99 )
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 June 30, 2019 2018 Common stock subject to repurchase — 32,361 Options and awards issued and outstanding 5,303,268 3,904,668

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Date of incorporation		Jun. 8, 2012
Accumulated deficit		$ 278,197		$ 202,553
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		271,224	$ 182,069
Cash, cash equivalents and short and long-term investments		602,400
Follow-On-Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock shares issued	5,663,750
Common stock shares issued, price per share	$ 51
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		$ 271,200
Over Allotment Option
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock shares issued	738,750

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)Segment	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of reportable and operating segment | Segment	1
Lessee, operating lease, existence of option to extend	true
Lessee, operating lease, existence of option to terminate	true
Operating lease, liability	$ 1,831
Deferred rent			$ 200
Operating lease right-of-use assets	1,756
ASU 842
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Operating lease, liability	1,831	$ 2,126
Operating lease, Right of use asset, before reclassification		2,100
Operating lease right-of-use assets	$ 1,756	$ 1,940

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Cash Reported in Condensed Consolidated Statements of Cash Flows (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 443,693	$ 246,122
Restricted cash included in prepaid expenses and other current assets	364
Restricted cash included in restricted cash and other	1,856	2,143
Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows	$ 445,913	$ 248,265	$ 347,113	$ 224,857

Sanofi License and Collaborat_3

Sanofi License and Collaboration Agreement - Additional Information (Details) - USD ($)

Aug. 31, 2014

Jan. 31, 2017

Jun. 30, 2018

Dec. 31, 2016

Jun. 30, 2019

Jun. 30, 2018

Dec. 31, 2018

Mar. 31, 2018

Dec. 31, 2018

Jul. 31, 2019

Jul. 18, 2019

Dec. 31, 2017

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Collaboration and license revenue

$ 6,639,000

$ 11,970,000

Deferred revenue

$ 21,588,000

21,588,000

$ 33,558,000

Contract assets

$ 0

Contract liabilities

0

Royalty Bearing Licenses | Subsequent Event

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Amount Payable to reacquire research and development asset

$ 80,000,000

Sanofi (Aventis Inc.) | Royalty Rights | Subsequent Event

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Amount Payable to reacquire research and development asset

80,000,000

$ 80,000,000

Collaborative Agreement | Sanofi (Aventis Inc.)

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received for research and development

$ 105,000,000

Collaboration and license revenue

$ 104,300,000

Type of Revenue [Extensible List]

myok:ResearchAndDevelopmentMember

Research and development payment received attributed to redeemable convertible preferred stock call option

$ 700,000

Upfront cash received under collaboration agreement

$ 35,000,000

Up-front equity investment

10,000,000

Additional equity investments received

$ 26,500,000

Eligible to receive one-time, non-refundable contingent payment

25,000,000

Project non-refundable continuation payment

45,000,000

Milestone payment received

25,000,000

$ 25,000,000

Continuation payment, amount received

$ 45,000,000

Upfront cash payment

34,300,000

Net fair value allocated to equity

700,000

Variable consideration

25,000,000

Fixed fee

$ 45,000,000

Transaction price for extended term

$ 45,000,000

Collaboration and license revenue

0

12,000,000

Deferred revenue

0

Reduction in research and development expenses due to RPP reimbursements

$ 18,500,000

$ 7,100,000

Collaborative Agreement | Sanofi (Aventis Inc.) | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Funding from approved in-kind research and clinical activities

45,000,000

Additional research and development funding for collaboration

$ 15,000,000

Termination Agreement [Member] | Sanofi (Aventis Inc.) | Royalty Rights | Subsequent Event

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Amount Payable to reacquire research and development asset

$ 80,000,000

Sanofi License and Collaborat_4

Sanofi License and Collaboration Agreement - Changes in Contract Liabilities (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 33,558
Deferred revenue, Additions	0
Deferred revenue, Deductions	(11,970)
Deferred revenue, Balance at End of Period	$ 21,588

Sanofi License and Collaborat_5

Sanofi License and Collaboration Agreement - Schedule of Receivables and Related Prepayments From Collaboration Partner (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Prepayment from collaboration partner for mavacamten, Balance at Beginning of Period	$ 12,973
Prepayment from collaboration partner for mavacamten, Balance at End of Period	2,256
Collaborative Agreement | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivable from collaboration partner, Balance at Beginning of Period	0	$ 1,013
Deductions	0	(1,013)
Receivable from collaboration partner, Balance at End of Period	0	0
Prepayment from collaboration partner for mavacamten, Balance at Beginning of Period	12,973	4,432
Payments received from Sanofi	7,777	11,809
Actual expenses incurred	(18,494)	(7,126)
Prepayment from collaboration partner for mavacamten, Balance at End of Period	$ 2,256	$ 9,115

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets
Assets fair value	$ 602,376	$ 393,906
Money market funds
Assets
Assets fair value	443,632	245,194
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	443,632	245,194
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	158,744	148,712
Fair Value Measurements on Recurring Basis | Money market funds | Level 1
Assets
Assets fair value	443,632	245,194
U.S. government agency obligations
Assets
Assets fair value	98,622	85,033
U.S. government agency obligations | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	98,622	85,033
Corporate securities
Assets
Assets fair value	60,122	63,679
Corporate securities | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	$ 60,122	$ 63,679

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Fair Value Measurement of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 601,655	$ 393,968
Unrealized Gain	721	98
Unrealized Loss		(160)
Fair Value	602,376	393,906
Cash equivalents (due within 90 days)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	443,632	245,194
Fair Value	443,632	245,194
Short-term investments (due within one year)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	114,486	68,656
Unrealized Gain	306
Unrealized Loss		(92)
Fair Value	114,792	68,564
Long-term investments (due between one and two years)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	43,537	80,118
Unrealized Gain	415	98
Unrealized Loss		(68)
Fair Value	$ 43,952	$ 80,148

Leases - Additional Information

Leases - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 30, 2018USD ($)ft²	Jun. 30, 2019USD ($)ft²Facility	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)ft²Facility	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
Aggregate future minimum rental payments due						$ 96,308
Operating lease rent expense		$ 700	$ 600	$ 1,400	$ 1,000
New Facility
Lessee Lease Description [Line Items]
Area of leased property | ft²	129,800
Rent commencement date	2020-01
Lease period	10 years
Additional period of extension in lease contract	10 years
Aggregate future minimum rental payments due	$ 93,200
Standby letter of credit	$ 1,900
Existing Facilities
Lessee Lease Description [Line Items]
Area of leased property | ft²		70,500		70,500
Facilities available for operating lease | Facility		3		3
Lease expiration date				Apr. 1, 2020

Leases - Schedule of Informatio

Leases - Schedule of Information Related to Operating Leases (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Lessee Lease Description [Line Items]
Operating lease right-of-use assets	$ 1,756
Operating lease liabilities - current	1,831
Operating lease, liability	1,831
ASU 842
Lessee Lease Description [Line Items]
Operating lease right-of-use assets	1,756	$ 1,940
Operating lease liabilities - current	1,831	2,126
Operating lease, liability	$ 1,831	$ 2,126
Weighted average remaining lease term (years)	8 months 12 days	1 year
Weighted average discount rate	6.00%	6.00%

Leases - Schedule of Informat_2

Leases - Schedule of Information Related to Operating Lease Activity (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Lease Cost [Abstract]
Operating lease right-of-use assets obtained in exchange for lease obligations	$ 1,095
Operating lease rental expense	1,415
Operating lease payments	$ 1,528

Leases - Future Annual Payments

Leases - Future Annual Payments of Operating Lease Liabilities (Details) $ in Thousands	Jun. 30, 2019USD ($)
Operating Lease Liabilities Payments Due [Abstract]
2019 (six months remaining)	$ 1,478
2020	387
Total future lease payments	1,865
Less: imputed interest	(34)
Total operating lease liabilities	$ 1,831

Leases - Schedule of Future Ann

Leases - Schedule of Future Annual Minimum Operating Lease Payments Due Under Leases for the New Facility (Details) $ in Thousands	Jun. 30, 2019USD ($)
Lessee Lease Description [Line Items]
2020	$ 387
Total future lease payments	1,865
New Facility
Lessee Lease Description [Line Items]
2020	5,454
2021	8,461
2022	8,757
2023	9,063
Thereafter	61,444
Total future lease payments	$ 93,179

Leases - Schedule of Future A_2

Leases - Schedule of Future Annual Minimum Operating Lease Payments Due Under Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Operating Leases [Abstract]
2019	$ 2,752
2020	5,831
2021	8,461
2022	8,757
2023	9,063
Thereafter	61,444
Total	$ 96,308

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment	$ 12,368	$ 11,127
Less: Accumulated depreciation	(6,933)	(5,989)
Property and equipment, net	5,435	5,138
Scientific Equipment
Property Plant And Equipment [Line Items]
Property and equipment	9,944	9,126
Furniture and Equipment
Property Plant And Equipment [Line Items]
Property and equipment	1,471	1,248
Capitalized Software
Property Plant And Equipment [Line Items]
Property and equipment	389	302
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	$ 564	$ 451

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Depreciation expense	$ 400	$ 400	$ 944	$ 723

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Clinical research and development	$ 16,568	$ 10,903
Payroll-related liabilities	6,919	8,151
Other	1,257	1,704
Total accrued liabilities	$ 24,744	$ 20,758

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Purchase commitment cancellation notice period	30 days
Contingent liability for accrual	$ 0	$ 0

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 2018
Class Of Stock [Line Items]
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		$ 271,224	$ 182,069
Follow-On-Offering
Class Of Stock [Line Items]
Common stock shares issued	5,663,750
Common stock shares issued, price per share	$ 51
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		$ 271,200
Over Allotment Option
Class Of Stock [Line Items]
Common stock shares issued	738,750

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Issuance (Details) - shares	Jun. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	7,422,244	5,549,908
Options and Awards Issued and Outstanding
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	5,303,268	3,864,407
2015 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	956,329	904,785
2015 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	1,162,647	780,716

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense Related to Equity Awards Granted to Employees and Consultants (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 8,638	$ 4,782	$ 15,619	$ 8,413
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	3,795	2,050	6,758	3,641
Selling, general and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 4,843	$ 2,732	$ 8,861	$ 4,772

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option and Other Equity Award Activity Under Plans (Details)	6 Months Ended
	Jun. 30, 2019$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Subject to Outstanding Options, Beginning Balance | shares	3,701,461
Shares Subject to Outstanding Options, granted | shares	1,179,253
Shares Subject to Outstanding Options, exercised | shares	(88,629)
Shares Subject to Outstanding Options, canceled | shares	(71,235)
Shares Subject to Outstanding Options, Ending Balance | shares	4,720,850
Weighted Average Exercise Price per Share, Beginning Balance | $ / shares	$ 26.40
Weighted Average Exercise Price Per Share, granted | $ / shares	41.34
Weighted Average Exercise Price per Share, exercised | $ / shares	11.23
Weighted Average Exercise Price Per Share, forfeited | $ / shares	32.24
Weighted Average Exercise Price Per Share, Ending Balance | $ / shares	$ 30.33
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Subject to Outstanding Awards, Beginning Balance | shares	162,946
Shares Subject to Outstanding Awards, RSUs awarded | shares	460,149
Shares Subject to Outstanding Awards, RSUs released | shares	(32,523)
Shares Subject to Outstanding Awards, RSUs forfeited | shares	(8,154)
Shares Subject to Outstanding Awards, Ending Balance | shares	582,418
Weighted Average Grant Date Fair Value. Beginning Balance | $ / shares	$ 53.37
Weighted Average Grant Date Fair Value. RSUs awarded | $ / shares	41.54
Weighted Average Grant Date Fair Value. RSUs released | $ / shares	52.06
Weighted Average Grant Date Fair Value. RSUs forfeited | $ / shares	47.25
Weighted Average Grant Date Fair Value. Ending Balance | $ / shares	$ 44.19

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	Apr. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized share based compensation expense		$ 8,638,000	$ 4,782,000	$ 15,619,000	$ 8,413,000
Performance Criteria
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized share based compensation expense		$ 0	$ 40,000	$ 0	$ 193,000
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares issued to participants in the 2015 ESPP	20,958

Net Loss per Share - Computatio

Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator
Net loss	$ (38,174)	$ (37,470)	$ (18,413)	$ (17,820)	$ (75,644)	$ (36,233)
Denominator
Weighted average shares outstanding	46,067,571		37,490,960		43,305,587	36,694,820
Less: weighted average shares subject to repurchase	(1,670)		(50,936)		(4,170)	(74,073)
Weighted average shares used to compute basic and diluted net loss per share	46,065,901		37,440,024		43,301,417	36,620,747
Net loss per share, basic and diluted	$ (0.83)		$ (0.49)		$ (1.75)	$ (0.99)

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Common stock subject to repurchase
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share		32,361
Options and Awards Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	5,303,268	3,904,668

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Details) - 2015 Employee Stock Purchase Plan - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Line Items]
ESPP contributions from plan participants	$ 0.3	$ 0.2
Contributions converted shares under benefit plan	8,000	5,000
Percentage of stock price at the beginning of offering period	85.00%	85.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Minimum percentage of likelihood of realization of tax benefits			50.00%
Income tax benefit	$ (218)	$ 0	$ (218)	$ 0

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Royalty Rights - Sanofi (Aventis Inc.) - Subsequent Event - USD ($) $ in Millions	Jul. 18, 2019	Jul. 31, 2019
Subsequent Event [Line Items]
Amount Payable to reacquire research and development asset	$ 80	$ 80
Royalty expense	50
Transferred to escrow	30
MYK-491 Program
Subsequent Event [Line Items]
Research and development asset reacquired	$ 4.3