Security	Date	Code	Shares	Price	A/D	Owned after	D/I
Common Stock^F1	Sep 8, 2017	S	200,350	$41.00	D	7,487,404	D
Common Stock^F2,F1	Sep 8, 2017	S	900	$42.388	D	7,486,504	D
Common Stock^F4,F5,F6	Sep 11, 2017	S	20,000	$44.254	D	57,651	D
Common Stock^F5,F7	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.
F2The price reported is a weighted average price. These shares were sold in multiple transactions at prices ranging from $42.20 to $42.45, inclusive. The reporting person 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).
F3Sale executed by Tepper.
F4The price reported is a weighted average price. These shares were sold in multiple transactions at prices ranging from $44.05 to $44.425, inclusive. The reporting person 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 (4).
F5Third 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.
F6The shares are directly held by Tepper.
F7The shares are directly held by Levin.