X0306 4 2017-10-10 0 0001649904 RHYTHM PHARMACEUTICALS, INC. RYTM 0001408591 THIRD ROCK VENTURES LP C/O THIRD ROCK VENTURES, LLC, 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001578976 Third Rock Ventures GP, L.P. 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001578973 TRV GP, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001180428 LEVIN MARK J 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001192603 STARR KEVIN P 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001192595 TEPPER ROBERT I 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 Common Stock 2017-10-10 4 C 0 3963810 A 3981437 D Series A Convertible Preferred Stock 2017-10-10 4 C 0 9479502 0.00 D Common Stock 1033751 0 D Series A-1 Junior Preferred Stock 2017-10-10 4 C 0 26868642 0.00 D Common Stock 2930059 0 D Represents the total number of shares of Common Stock received by Third Rock Ventures, L.P. ("TRV") upon conversion of the Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock of the Issuer in connection with the closing of the Issuer's initial public offering. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date. These shares are directly held by TRV. The general partner of TRV is Third Rock Ventures GP, L.P. ("TRV GP"). The general partner of TRV GP is TRV GP, LLC ("TRV GP LLC"). The individual managers of TRV GP LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("Tepper"). Each of TRV GP, TRV GP LLC, Levin, Starr and Tepper disclaims beneficial ownership of these 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. /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC, general partner of Third Rock Ventures GP, L.P., general partner of Third Rock Ventures, L.P. 2017-10-11 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC, general partner of Third Rock Ventures GP, L.P. 2017-10-11 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC 2017-10-11 /s/ Kevin Gillis by power of attorney for Mark Levin 2017-10-11 /s/ Kevin Gillis by power of attorney for Kevin Starr 2017-10-11 /s/ Kevin Gillis by power of attorney for Dr. Robert Tepper 2017-10-11