Security	Date	Code	Shares	Price	A/D	Owned after	D/I
Common Stock^F1,F2	Oct 18, 2017	J	1,000,000	—	D	3,320,348	D
Common Stock^F3	holding	—	—		—	116,927	D
Common Stock^F4	holding	—	—		—	145,286	D
Common Stock^F5	holding	—	—		—	85,509	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, L.P. ("TRV"), TRV distributed on October 18, 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, L.P. ("TRV GP"), the general partner of TRV, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP distributed, for no consideration, the Shares it received in the distribution by TRV 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. The general partner of TRV is 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 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 Starr. Includes Shares received in the distributions described in footnote (1) above.
F4These shares are directly held by Levin. Includes Shares received in the distributions described in footnote (1) above.
F5These shares are directly held by Tepper. Includes Shares received in the distributions described in footnote (1) above.