Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	Owned after	D/I
Series A Preferred Stock^F3,F2	—	Nov 6, 2017	C	6,122,448	D	—	—	Common Stock	1,466,805	0	D
Series B Preferred Stock^F3,F2	—	Nov 6, 2017	C	4,629,630	D	—	—	Common Stock	1,109,159	0	D

Explanation of responses

F1Represents the total number of shares of Common Stock received by Third Rock Ventures II, L.P. ("TRV II") upon conversion of the Series A Preferred Stock and Series B Preferred Stock of the Issuer in connection with the closing of the Issuer's initial public offering.
F2The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series 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.
F3These shares are directly held by 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 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.
F4Represents shares of Common Stock purchased by TRV II in the Issuer's initial public offering.