Security	Date	Code	Shares	Price	A/D	Owned after	D/I
Common Stock^F1	Mar 23, 2017	S	1,000,000	$17.0002	D	5,820,348	D
Common Stock^F2,F1	Mar 27, 2017	S	650,000	$18.0003	D	5,170,348	D
Common Stock^F3	holding	—	—		—	59,776	D
Common Stock^F4	holding	—	—		—	59,776	D
Common Stock^F5	holding	—	—		—	59,777	D

Table II — derivative securities

No Table II lines on this filing.

Explanation of responses

F1These 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"). 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.
F2This price is a weighted average price. These shares were sold in multiple transactions at prices ranging from $18.00 to $18.45, inclusive. Each of the reporting persons undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (2).
F3These shares are directly held by Starr.
F4These shares are directly held by Levin.
F5These shares are directly held by Tepper.