Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	Owned after	D/I
Series B-1 Preferred Stock^F3,F1,F2	—	Oct 14, 2015	C	8,105,314	D	—	—	Common Stock	8,105,314	0	D
Series C Preferred Stock^F3,F1,F2	—	Oct 14, 2015	C	565,037	D	—	—	Common Stock	565,037	0	D

Explanation of responses

F1The aggregate number of shares of Issuer's Preferred Stock held by the Reporting Person converted into Common Stock immediately prior to the closing of the Issuer's initial public offering and has no expiration date. These shares converted into Common Stock on a one-for-one basis.
F2Reflects the conversion of the Issuer's Preferred Stock into Common Stock on a one-for-one basis, which became effective immediately prior to the closing of the Issuer's initial public offering.
F3The shares are directly held by Third Rock Ventures, L.P. ("TRV"). The general partner of TRV is Third Rock Ventures GP, L.P. ("TRV GP LP"). The general partner of TRV GP LP 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") and, as such, each of TRV GP LP and TRV GP LLC, Levin, Starr and Tepper exercises shared voting and investment power over the shares held of record by TRV. Each of the Reporting Persons disclaims beneficial ownership of the shares except to the extent of their pecuniary interest therein, if any.