Security	Date	Code	Shares	Price	A/D	Owned after	D/I
Common Stock^F1	Sep 25, 2017	S	201,400	$43.00	D	7,285,104	D
Common Stock^F1	Sep 26, 2017	S	400,000	$43.00	D	6,885,104	D
Common Stock^F2,F3	holding	—	—		—	57,651	D
Common Stock^F2,F4	holding	—	—		—	77,652	D

Table II — derivative securities

No Table II lines on this filing.

Explanation of responses

F1These 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"). 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.
F2Third Rock Ventures III, L.P. ("TRV III") directly holds 1,496,598 shares of Common Stock of the Issuer. The general partner of TRV III is Third Rock Ventures GP III, L.P. ("TRV GP III"). The general partner of TRV GP III is TRV GP III, LLC ("TRV GP III LLC"). The individual managers of TRV GP III LLC are Levin, Starr and Tepper. Each of TRV GP III, TRV GP III 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.
F3The shares are directly held by Tepper.
F4The shares are directly held by Levin.