X0306 4 2018-01-03 0 0001552451 MyoKardia Inc MYOK 0001569705 Third Rock Ventures III, L.P. C/O THIRD ROCK VENTURES, LLC, 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001637300 Third Rock Ventures GP III, L.P. 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001637299 TRV GP III, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001180428 LEVIN MARK J C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001192595 TEPPER ROBERT I C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 Common Stock 2018-01-03 4 S 0 816326 42.0001 D 680272 D Common Stock 201042 D Common Stock 221042 D The price reported is a weighted average price. These shares were sold in multiple transactions at prices ranging from $42.00 to $42.15, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (1). These shares are directly held by Third Rock Ventures III, L.P. ("TRV III"). 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 Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("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. Third Rock Ventures II, L.P. ("TRV II") directly holds 2,885,104 shares of Common Stock of the Issuer. The general partner of TRV II is Third Rock Ventures GP II, L.P. ("TRV GP II"). The general partner of TRV GP II is TRV GP II, LLC ("TRV GP II LLC"). The individual managers of TRV GP II LLC are Levin, Starr and 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. The shares are directly held by Levin. The shares are directly held by Tepper. /s/ Kevin Gillis, Chief Financial Officer of TRV GP III, LLC, general partner of Third Rock Ventures GP III, L.P., general partner of Third Rock Ventures III, L.P. 2018-01-05 /s/ Kevin Gillis, Chief Financial Officer of TRV GP III, LLC, general partner of Third Rock Ventures GP III, L.P. 2018-01-05 /s/ Kevin Gillis, Chief Financial Officer of TRV GP III, LLC 2018-01-05 /s/ Kevin Gillis by power of attorney for Mark Levin 2018-01-05 /s/ Kevin Gillis by power of attorney for Dr. Robert Tepper 2018-01-05