Tepper Robert I

SEC source: 0001104659-18-043854 · a4.xml · derivative · entry 3

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote

Disclosed balance after: 0

Underlying: Common Stock · 0 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F3 · derivative.exercise_price: All series of Preferred Stock automatically converted into the Issuer's Common Stock on a 5-for-1 basis upon the closing of the Issuer's initial public offering on June 29, 2018 and had no expiration date.

F1 · shares: Third Rock Ventures III, L.P. ("TRV III") received 8,999,999 shares of Common Stock of the Issuer upon conversion of 45,000,000 shares of Series A Preferred Stock in connection with the closing of the Issuer's initial public offering.

F3 · price: All series of Preferred Stock automatically converted into the Issuer's Common Stock on a 5-for-1 basis upon the closing of the Issuer's initial public offering on June 29, 2018 and had no expiration date.

F3 · derivative.exercise_date: All series of Preferred Stock automatically converted into the Issuer's Common Stock on a 5-for-1 basis upon the closing of the Issuer's initial public offering on June 29, 2018 and had no expiration date.

F3 · derivative.expiration_date: All series of Preferred Stock automatically converted into the Issuer's Common Stock on a 5-for-1 basis upon the closing of the Issuer's initial public offering on June 29, 2018 and had no expiration date.

F1 · derivative.underlying_shares: Third Rock Ventures III, L.P. ("TRV III") received 8,999,999 shares of Common Stock of the Issuer upon conversion of 45,000,000 shares of Series A Preferred Stock in connection with the closing of the Issuer's initial public offering.

F5 · ownership.nature: These shares are directly held by TRV III. 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 Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("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.

SEC source: 0001104659-18-043854 · a4.xml · nonDerivative · entry 2

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote

Disclosed balance after: 0

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F2 · shares: TRV III received 341,637 shares of Common Stock of the Issuer upon conversion of 1,708,185 shares of Series B Preferred Stock in connection with the closing of the Issuer's initial public offering.

F3 · price: All series of Preferred Stock automatically converted into the Issuer's Common Stock on a 5-for-1 basis upon the closing of the Issuer's initial public offering on June 29, 2018 and had no expiration date.

F6 · balance_after: After the above transaction, TRV III owned 9,741,636 shares of Common Stock of the Issuer.

F5 · ownership.nature: These shares are directly held by TRV III. 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 Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("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.

SEC source: 0001104659-18-043854 · a4.xml · nonDerivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote

Disclosed balance after: 0

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · shares: Third Rock Ventures III, L.P. ("TRV III") received 8,999,999 shares of Common Stock of the Issuer upon conversion of 45,000,000 shares of Series A Preferred Stock in connection with the closing of the Issuer's initial public offering.

F3 · price: All series of Preferred Stock automatically converted into the Issuer's Common Stock on a 5-for-1 basis upon the closing of the Issuer's initial public offering on June 29, 2018 and had no expiration date.

F4 · balance_after: After the above transaction, TRV III owned 9,399,999 shares of Common Stock of the Issuer.

F5 · ownership.nature: These shares are directly held by TRV III. 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 Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("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.

SEC source: 0000899243-18-017587 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures III, L.P. · CIK 0001569705 · 10% owner

Third Rock Ventures GP III, L.P. · CIK 0001637300 · 10% owner

TRV GP III, LLC · CIK 0001637299 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 7,891,176

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures III, L.P. ("TRV III"), Third Rock Ventures GP III, L.P. ("TRV GP III") and TRV GP III, LLC ("TRV GP III LLC"), TRV III distributed on June 20, 2018, for no consideration, 1,500,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to TRV GP III, the general partner of TRV III, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP III distributed, for no consideration, the Shares it received in the distribution by TRV III 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures III, L.P. ("TRV III"), Third Rock Ventures GP III, L.P. ("TRV GP III") and TRV GP III, LLC ("TRV GP III LLC"), TRV III distributed on June 20, 2018, for no consideration, 1,500,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to TRV GP III, the general partner of TRV III, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP III distributed, for no consideration, the Shares it received in the distribution by TRV III 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.

F2 · ownership.direct: These shares are directly held by TRV III. The general partner of TRV III is TRV GP III. The general partner of TRV GP III is TRV GP III LLC. The individual managers of TRV GP III LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("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.

SEC source: 0001640455-18-000081 · wf-form4_152961560410969.xml · derivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 13,550

Underlying: Common Stock · 13,550 shares

Expires 2028-06-19

F1 · derivative.exercise_date: The option will vest in equal quarterly installments during the four quarters following the grant date, subject to the Reporting Person's continued service on the Issuer's board of directors.

SEC source: 0001209191-18-037229 · doc4.xml · derivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 10,182

Underlying: Common Stock · 10,182 shares

Expires 2028-06-07

F1 · derivative.exercise_date: Stock option shall vest in full (i.e., in single installment) upon the earlier to occur of the first anniversary of the date of grant or the date of the next annual meeting of shareholders.

SEC source: 0000899243-18-006627 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 885,104

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on March 7, 2018, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on March 7, 2018, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F2 · ownership.direct: These 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. 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.

SEC source: 0000899243-18-000584 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures III, L.P. · CIK 0001569705 · 10% owner

Third Rock Ventures GP III, L.P. · CIK 0001637300 · 10% owner

TRV GP III, LLC · CIK 0001637299 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 680,272

F1 · price: The price reported is a weighted average price. These shares were sold in multiple transactions at prices ranging from $42.00 to $42.15, 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 (1).

F2 · ownership.direct: These shares are directly held by Third Rock Ventures III, L.P. ("TRV III"). 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 Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("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.

SEC source: 0000899243-17-028251 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 2,885,104

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on December 6, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on December 6, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F2 · ownership.direct: These 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. 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.

SEC source: 0000899243-17-025454 · doc4.xml · derivative · entry 4

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 0

Underlying: Common Stock · 1,109,159 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F2 · derivative.exercise_price: The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.exercise_date: The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.expiration_date: The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F3 · ownership.direct: These shares are directly held by 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 these 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.

SEC source: 0000899243-17-025454 · doc4.xml · derivative · entry 3

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 0

Underlying: Common Stock · 1,466,805 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F2 · derivative.exercise_price: The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.exercise_date: The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.expiration_date: The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F3 · ownership.direct: These shares are directly held by 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 these 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.

SEC source: 0000899243-17-025454 · doc4.xml · nonDerivative · entry 2

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 2,790,964

F4 · kind: Represents shares of Common Stock purchased by TRV II in the Issuer's initial public offering.

F3 · ownership.direct: These shares are directly held by 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 these 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.

SEC source: 0000899243-17-025454 · doc4.xml · nonDerivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · Director · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 2,575,964

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · shares: Represents the total number of shares of Common Stock received by Third Rock Ventures II, L.P. ("TRV II") upon conversion of the Series A Preferred Stock and Series B Preferred Stock of the Issuer in connection with the closing of the Issuer's initial public offering.

F2 · price: The Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 4.174-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F3 · ownership.direct: These shares are directly held by 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 these 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.

SEC source: 0000899243-17-024464 · doc4.xml · nonDerivative · entry 1

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 3,320,348

F1 · kind: Pursuant 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.

F1 · price: Pursuant 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.

F2 · ownership.direct: These 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.

SEC source: 0000899243-17-024006 · doc4.xml · derivative · entry 3

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 0

Underlying: Common Stock · 2,930,059 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F2 · derivative.exercise_price: The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.exercise_date: The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.expiration_date: The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F3 · ownership.direct: These shares are directly held by 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 these 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.

SEC source: 0000899243-17-024006 · doc4.xml · derivative · entry 2

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 0

Underlying: Common Stock · 1,033,751 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F2 · derivative.exercise_price: The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.exercise_date: The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F2 · derivative.expiration_date: The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F3 · ownership.direct: These shares are directly held by 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 these 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.

SEC source: 0000899243-17-024006 · doc4.xml · nonDerivative · entry 1

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 3,981,437

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · shares: Represents the total number of shares of Common Stock received by Third Rock Ventures, L.P. ("TRV") upon conversion of the Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock of the Issuer in connection with the closing of the Issuer's initial public offering.

F2 · price: The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock converted into Common Stock on a 9.17-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date.

F3 · ownership.direct: These shares are directly held by 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 these 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.

SEC source: 0000899243-17-023514 · doc4.xml · nonDerivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 29,516

F1 · kind: The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan previously adopted by the Reporting Person.

SEC source: 0000899243-17-022948 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 4,885,104

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on September 28, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on September 28, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F2 · ownership.direct: These 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. 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.

SEC source: 0000899243-18-003665 · doc4a.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 4,885,104

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on September 28, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on September 28, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F2 · ownership.direct: These 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. 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.

SEC source: 0000899243-17-022639 · doc4.xml · nonDerivative · entry 2

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 6,885,104

F1 · ownership.direct: These 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.

SEC source: 0000899243-17-022639 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 7,285,104

F1 · ownership.direct: These 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.

SEC source: 0000899243-17-022406 · doc4.xml · nonDerivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 54,293

F1 · kind: The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan previously adopted by the Reporting Person.

SEC source: 0000899243-17-021893 · doc4.xml · nonDerivative · entry 3

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 57,651

F3 · kind: Sale executed by Tepper.

F4 · price: The 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).

F5 · balance_after: Third 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.

F5 · ownership.direct: Third 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.

F6 · ownership.direct: The shares are directly held by Tepper.

SEC source: 0000899243-17-021893 · doc4.xml · nonDerivative · entry 2

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 7,486,504

F2 · price: The 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).

F1 · ownership.direct: These 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.

SEC source: 0000899243-17-021893 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 7,487,404

F1 · ownership.direct: These 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.

SEC source: 0001179110-17-010702 · edgar.xml · derivative · entry 6

TEPPER ROBERT I · CIK 0001192595 · Director

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote.

Disclosed balance after: 33,333

Underlying: Common Stock · 33,333 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F2 · derivative.exercise_date: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F2 · derivative.expiration_date: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F4 · ownership.nature: These shares are held by TRV. The general partner of TRV is TRV GP. The general partner of TRV GP is TRV GP LLC. The individual managers of TRV GP LLC are Levin, Starr and 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.

SEC source: 0001179110-17-010702 · edgar.xml · derivative · entry 5

TEPPER ROBERT I · CIK 0001192595 · Director

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote.

Disclosed balance after: 0

Underlying: Series B Preferred Stock · 173,611 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F2 · derivative.exercise_price: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F2 · shares: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F2 · derivative.exercise_date: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F2 · derivative.expiration_date: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F2 · derivative.underlying_shares: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F2 · balance_after: The Series B Preferred Stock Warrant was exercisable for such shares of Preferred Stock at an exercise price of $1.44 per share. Upon the closing of the Issuer's initial public offering, the Warrant automatically became exercisable for 33,333 shares of Common Stock at an exercise price of $7.50 per share. The Warrant is exercisable at any time at the holder's election on or before April 16, 2021.

F3 · ownership.nature: These shares were 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.

SEC source: 0001179110-17-010702 · edgar.xml · derivative · entry 4

TEPPER ROBERT I · CIK 0001192595 · Director

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote.

Disclosed balance after: 0

Underlying: Common Stock · 435,029 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F1 · derivative.exercise_date: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F1 · derivative.expiration_date: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F3 · ownership.nature: These shares were 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.

SEC source: 0001179110-17-010702 · edgar.xml · derivative · entry 3

TEPPER ROBERT I · CIK 0001192595 · Director

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote.

Disclosed balance after: 0

Underlying: Common Stock · 480,003 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F1 · derivative.exercise_date: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F1 · derivative.expiration_date: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F3 · ownership.nature: These shares were 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.

SEC source: 0001179110-17-010702 · edgar.xml · derivative · entry 2

TEPPER ROBERT I · CIK 0001192595 · Director

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote.

Disclosed balance after: 0

Underlying: Common Stock · 706,060 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F1 · derivative.exercise_date: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F1 · derivative.expiration_date: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F3 · ownership.nature: These shares were 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.

SEC source: 0001179110-17-010702 · edgar.xml · nonDerivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · Director

10b5-1 filing: Not disclosed

Ownership: Indirect · See footnote.

Disclosed balance after: 1,621,092

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock converted into Common Stock on a 5.2083-for-one-basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares had no expiration date.

F4 · ownership.nature: These shares are held by TRV. The general partner of TRV is TRV GP. The general partner of TRV GP is TRV GP LLC. The individual managers of TRV GP LLC are Levin, Starr and 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.

SEC source: 0001179110-17-010597 · edgar.xml · derivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · Director

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 12,960

Underlying: Common Stock · 12,960 shares

Expires 2027-07-18

F1 · derivative.exercise_date: This option was granted, effective on July 19, 2017 and vests over one year, with 8.33% of the shares underlying the option vesting on August 19, 2017, and an addition 8.33% of the shares underlying the option vesting at the end of each successive one-month period thereafter.

SEC source: 0000899243-17-018238 · doc4.xml · nonDerivative · entry 1

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 4,320,348

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures, L.P. ("TRV"), TRV distributed on July 13, 2017, for no consideration, 850,000 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures, L.P. ("TRV"), TRV distributed on July 13, 2017, for no consideration, 850,000 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.

F2 · ownership.direct: These 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.

SEC source: 0000899243-17-017617 · doc4.xml · nonDerivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 44,777

F1 · kind: The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan previously adopted by the Reporting Person.

SEC source: 0000899243-17-016554 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 2,475,191

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on June 14, 2017, for no consideration, 1,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on June 14, 2017, for no consideration, 1,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F2 · ownership.direct: These 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. 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.

SEC source: 0000899243-17-013908 · doc4.xml · nonDerivative · entry 3

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 3,475,191

F1 · kind: Reflects gifts by Mark Levin ("Levin"). The remaining shares are directly held by Levin.

F2 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on May 17, 2017, for no consideration, 1,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F3 · ownership.direct: These 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. The individual managers of TRV GP II LLC are 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.

SEC source: 0000899243-17-013908 · doc4.xml · nonDerivative · entry 2

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 163,996

F1 · shares: Reflects gifts by Mark Levin ("Levin"). The remaining shares are directly held by Levin.

F1 · ownership.direct: Reflects gifts by Mark Levin ("Levin"). The remaining shares are directly held by Levin.

SEC source: 0000899243-17-010312 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures III, L.P. · CIK 0001569705 · 10% owner

Third Rock Ventures GP III, L.P. · CIK 0001637300 · 10% owner

TRV GP III, LLC · CIK 0001637299 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 9,391,176

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures III, L.P. ("TRV III"), Third Rock Ventures GP III, L.P. ("TRV GP III") and TRV GP III, LLC ("TRV GP III LLC"), TRV III distributed on April 12, 2017, for no consideration, 920,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to TRV GP III, the general partner of TRV III, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP III distributed, for no consideration, the Shares it received in the distribution by TRV III 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures III, L.P. ("TRV III"), Third Rock Ventures GP III, L.P. ("TRV GP III") and TRV GP III, LLC ("TRV GP III LLC"), TRV III distributed on April 12, 2017, for no consideration, 920,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to TRV GP III, the general partner of TRV III, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP III distributed, for no consideration, the Shares it received in the distribution by TRV III 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.

F2 · ownership.direct: These shares are directly held by TRV III. The general partner of TRV III is TRV GP III. The general partner of TRV GP III is TRV GP III LLC. The individual managers of TRV GP III LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("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.

SEC source: 0000899243-17-013908 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 165,704

F1 · shares: Reflects gifts by Mark Levin ("Levin"). The remaining shares are directly held by Levin.

F1 · ownership.direct: Reflects gifts by Mark Levin ("Levin"). The remaining shares are directly held by Levin.

SEC source: 0000899243-17-009646 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 4,475,191

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on April 6, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on April 6, 2017, for no consideration, 2,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F2 · ownership.direct: These 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. 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.

SEC source: 0000899243-17-009893 · doc4.xml · nonDerivative · entry 1

TEPPER ROBERT I · CIK 0001192595 · 10% owner

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 229,773

F1 · price: The price reported is a weighted average price. These shares were sold in multiple transactions at prices ranging from $33.00 to $33.40, 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 (1).

SEC source: 0000899243-17-009104 · doc4.xml · nonDerivative · entry 1

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 7,687,754

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on March 30, 2017, for no consideration, 850,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures II, L.P. ("TRV II"), Third Rock Ventures GP II, L.P. ("TRV GP II") and TRV GP II, LLC ("TRV GP II LLC"), TRV II distributed on March 30, 2017, for no consideration, 850,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to 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.

F2 · ownership.direct: These 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. 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.

SEC source: 0000899243-17-008731 · doc4.xml · nonDerivative · entry 2

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 5,170,348

F2 · price: This 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).

F1 · ownership.direct: These 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.

SEC source: 0000899243-17-008731 · doc4.xml · nonDerivative · entry 1

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 5,820,348

F1 · ownership.direct: These 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.

SEC source: 0000899243-17-007785 · doc4.xml · nonDerivative · entry 1

THIRD ROCK VENTURES LP · CIK 0001408591 · 10% owner

Third Rock Ventures GP, L.P. · CIK 0001578976 · 10% owner

TRV GP, LLC · CIK 0001578973 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 6,820,348

F1 · kind: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures, L.P. ("TRV"), TRV distributed on March 15, 2017, for no consideration, 850,000 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.

F1 · price: Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures, L.P. ("TRV"), TRV distributed on March 15, 2017, for no consideration, 850,000 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.

F2 · ownership.direct: These 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.

SEC source: 0000899243-17-006623 · doc4.xml · nonDerivative · entry 5

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

Third Rock Ventures III, L.P. · CIK 0001569705 · 10% owner

Third Rock Ventures GP III, L.P. · CIK 0001637300 · 10% owner

TRV GP III, LLC · CIK 0001637299 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 146,208

F6 · price: This price is a weighted average price. These shares were sold in multiple transactions at prices ranging from $28.50 to $28.975, inclusive.

F4 · ownership.direct: These shares are directly held by Levin.

SEC source: 0000899243-17-006623 · doc4.xml · nonDerivative · entry 4

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

Third Rock Ventures III, L.P. · CIK 0001569705 · 10% owner

Third Rock Ventures GP III, L.P. · CIK 0001637300 · 10% owner

TRV GP III, LLC · CIK 0001637299 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 157,108

F5 · price: This price is a weighted average price. These shares were sold in multiple transactions at prices ranging from $27.35 to $28.275, inclusive.

F4 · ownership.direct: These shares are directly held by Levin.

SEC source: 0000899243-17-006623 · doc4.xml · nonDerivative · entry 3

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

Third Rock Ventures III, L.P. · CIK 0001569705 · 10% owner

Third Rock Ventures GP III, L.P. · CIK 0001637300 · 10% owner

TRV GP III, LLC · CIK 0001637299 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 164,387

F3 · price: This price is a weighted average price. These shares were sold in multiple transactions at prices ranging from $26.35 to $27.30, inclusive. Levin 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 footnotes (3), (5) and (6) to this Form 4.

F4 · ownership.direct: These shares are directly held by Levin.

SEC source: 0000899243-17-006623 · doc4.xml · nonDerivative · entry 2

Third Rock Ventures II, L.P. · CIK 0001488495 · 10% owner

Third Rock Ventures GP II, L.P. · CIK 0001614061 · 10% owner

TRV GP II, LLC · CIK 0001614073 · 10% owner

Third Rock Ventures III, L.P. · CIK 0001569705 · 10% owner

Third Rock Ventures GP III, L.P. · CIK 0001637300 · 10% owner

TRV GP III, LLC · CIK 0001637299 · 10% owner

LEVIN MARK J · CIK 0001180428 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

STARR KEVIN P · CIK 0001192603 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 285,713

F2 · ownership.direct: These shares are directly held by Third Rock Ventures III, L.P. ("TRV III"). 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.