Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	Owned after	D/I
Series A Convertible Preferred Stock^F3,F2	—	Oct 10, 2017	C	9,479,502	D	—	—	Common Stock	1,033,751	0	D
Series A-1 Junior Preferred Stock^F3,F2	—	Oct 10, 2017	C	26,868,642	D	—	—	Common Stock	2,930,059	0	D

Explanation of responses

F1Represents 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.
F2The 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.
F3These 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.