Security	Date	Code	Shares	Price	A/D	Owned after	D/I
Common Stock^F1,F2	May 5, 2015	C	5,454,545	—	A	5,727,272	D
Common Stock^F1,F2	May 5, 2015	C	1,424,242	—	A	7,151,514	D
Common Stock^F1,F2	May 5, 2015	C	241,545	—	A	7,393,059	D

Table II — derivative securities

Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	D/I
Series A Preferred Stock^F1	—	May 5, 2015	C	30,000,000	D	—	—	Common Stock	5,454,545	D
Series B Preferred Stock^F1	—	May 5, 2015	C	7,833,333	D	—	—	Common Stock	1,424,242	D
Series C Preferred Stock^F1	—	May 5, 2015	C	1,328,502	D	—	—	Common Stock	241,545	D

Explanation of responses

F1On May 5, 2015, in connection with the completion of the issuer's initial public offering, all shares of Convertible Preferred Stock converted into Common Stock on a 5.5-for-1 basis for no additional consideration. The Preferred Stock had no expiration date.
F2The shares are directly held by Third Rock Ventures II, L.P. ("TRV II"). The general partner of TRV II is Third Rock Ventures GP II, L.P. ("TRV GP II LP"). The general partner of TRV GP II LP 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") and, as such, each of TRV GP II LP and TRV GP II LLC, Levin, Starr and Tepper exercises shared voting and investment power over the shares held of record by TRV II. Each of the Reporting Persons disclaims beneficial ownership of the shares except to the extent of their pecuniary interest therein, if any.