Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	D/I
Series A-1 Preferred Stock^F4,F2	—	Feb 8, 2016	C	6,628,542	D	—	—	Common Stock	2,549,437	D
Series A-2 Preferred Stock^F4,F2	—	Feb 8, 2016	C	5,255,300	D	—	—	Common Stock	2,021,269	D
Series B Preferred Stock^F4,F2	—	Feb 8, 2016	C	222,222	D	—	—	Common Stock	85,470	D

Explanation of responses

F1Represents the total number of shares of Common Stock received by Third Rock Ventures III, L.P. ("TRV III") upon conversion of Series A-1, Series A-2 and Series B Preferred Stock in connection with the closing of the Issuer's initial public offering.
F2The Series A-1, Series A-2 and Series B Preferred Stock converted into Common Stock on a 2.6-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A-1, Series A-2 and B 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.
F3The shares are directly held by 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 these shares except to the extent of its or his pecuniary interest therein, if any.
F4Each of TRV GP III, TRV GP III LLC, Levin, Starr and Tepper disclaims beneficial ownership of these shares except to the extent of its or his pecuniary interest therein, if any.