X0306 4 2017-09-25 0 0001552451 MyoKardia Inc MYOK 0001192603 STARR KEVIN P C/O MYOKARDIA, INC. 333 ALLERTON AVENUE SOUTH SAN FRANCISCO CA 94080 1 0 1 0 Common Stock 2017-09-25 4 S 0 201400 43.00 D 7285104 I See Footnote Common Stock 2017-09-26 4 S 0 400000 43.00 D 6885104 I See Footnote Common Stock 77652 D These shares are directly held by Third Rock Ventures II, L.P. ("TRV II"). 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 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. 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. The shares are directly held by Starr. /s/ Kevin Gillis by power of attorney for Kevin Starr 2017-09-27