Sec Form 4 Filing - Third Rock Ventures II, L.P. @ MyoKardia Inc - 2017-03-30

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Third Rock Ventures II, L.P. 2. Issuer Name and Ticker or Trading Symbol MyoKardia Inc [ MYOK] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O THIRD ROCK VENTURES, LLC, 29 NEWBURY STREET, 3RD FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 03/30/2017 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 03/30/2017 J( 1 ) 850,000 D 7,687,754 D ( 2 ) Common Stock 77,652 ( 4 ) D ( 3 ) ( 4 ) Common Stock 77,651 ( 5 ) D ( 3 ) ( 5 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Third Rock Ventures II, L.P. C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON, MA02116 X Third Rock Ventures GP II, L.P. C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON, MA02116 X TRV GP II, LLC C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON, MA02116 X LEVIN MARK J C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON, MA02116 X TEPPER ROBERT I C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON, MA02116 X Signatures /s/ Kevin Gillis, Chief Financial Officer of TRV GP II, LLC, general partner of Third Rock Ventures GP II, L.P., general partner of Third Rock Ventures II, L.P. 04/03/2017 Signature of Reporting Person Date /s/ Kevin Gillis, Chief Financial Officer of TRV GP II, LLC, general partner of Third Rock Ventures GP II, L.P. 04/03/2017 Signature of Reporting Person Date /s/ Kevin Gillis, Chief Financial Officer of TRV GP II, LLC 04/03/2017 Signature of Reporting Person Date /s/ Kevin Gillis by power of attorney for Mark Levin 04/03/2017 Signature of Reporting Person Date /s/ Kevin Gillis by power of attorney for Dr. Robert Tepper 04/03/2017 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on March 30, 2017, for no consideration, 850,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to TRV GP II, the general partner of TRV II, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP II distributed, for no consideration, the Shares it received in the distribution by TRV II to its partners, representing each such partner's pro rata interest in such Shares. All of the aforementioned distributions were made in accordance with the exemptions afforded by Rules 16a-13 and 16a-9 of the Securities Exchange Act of 1934, as amended. ( 2 )These shares are directly held by TRV II. The general partner of TRV II is TRV GP II. The general partner of TRV GP II is TRV GP II LLC. The individual managers of TRV GP II LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("Tepper"). Each of TRV GP II, TRV GP II LLC, Levin, Starr and Tepper disclaims beneficial ownership of the shares except to the extent of its or his pecuniary interest therein, if any, and this report shall not be deemed an admission that it or he is the beneficial owner of such shares. ( 3 )Third Rock Ventures III, L.P. ("TRV III") directly holds 1,496,598 shares of Common Stock of the Issuer. The general partner of TRV III is Third Rock Ventures GP III, L.P. ("TRV GP III"). The general partner of TRV GP III is TRV GP III, LLC ("TRV GP III LLC"). The individual managers of TRV GP III LLC are Levin, Starr and Tepper. Each of TRV GP III, TRV GP III LLC, Levin, Starr and Tepper disclaims beneficial ownership of the shares except to the extent of its or his pecuniary interest therein, if any, and this report shall not be deemed an admission that it or he is the beneficial owner of such shares. ( 4 )The shares are directly held by Levin. Includes Shares received in the distributions described in footnote (1) above. ( 5 )The shares are directly held by Tepper. Includes Shares received in the distributions described in footnote (1) above.

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