Security	Date	Code	Shares	Price	A/D	Owned after	D/I
Common Stock^F1,F2	Feb 27, 2017	J	1,000,000	—	D	3,453,753	D
Common Stock^F3	holding	—	—		—	141,240	D
Common Stock^F4	holding	—	—		—	141,244	D
Common Stock^F5	holding	—	—		—	34,750	D

Table II — derivative securities

No Table II lines on this filing.

Explanation of responses

F1Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), TRV II distributed on February 27, 2017, for no consideration, 1,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to Third Rock Ventures GP II, L.P. ("TRV GP II"), the general partner of TRV II, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP II distributed, for no consideration, the Shares it received in the distribution by TRV II to its partners, representing each such partner's pro rata interest in such Shares. All of the aforementioned distributions were made in accordance with the exemptions afforded by Rules 16a-13 and 16a-9 of the Securities Exchange Act of 1934, as amended.
F2These shares are directly held by TRV II. The general partner of TRV II is 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 the 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.
F3These shares are directly held by Tepper.
F4These shares are directly held by Levin.
F5These shares are directly held by Starr.