Third Rock Ventures III, L.P.

SEC source: 0000899243-20-019850 · doc4.xml · nonDerivative · entry 2

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

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Third Rock Ventures IV, L.P. · CIK 0001688327 · 10% owner

Third Rock Ventures GP IV, L.P. · CIK 0001688326 · 10% owner

TRV GP IV, LLC · CIK 0001688325 · 10% owner

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

10b5-1 filing: Not disclosed

Ownership: Indirect · See Footnotes

Disclosed balance after: 2,793,770

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 · ownership.nature: Each share of Series A Preferred Stock automatically converted into shares of the Issuer's common stock, par value $0.001 per share ("Common Stock") on a 1-for-3.55092 basis upon the closing of the Issuer's initial public offering on July 20, 2020 and had no expiration date.

F4 · ownership.nature: These shares are directly held by Third Rock Ventures IV, L.P. ("TRV IV").

F5 · ownership.nature: The general partner of TRV IV is Third Rock Ventures GP IV, L.P. ("TRV GP IV"). The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Each of the reporting persons, except for TRV IV, disclaims beneficial ownership of these shares except to the extent of its 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.

F7 · ownership.nature: Each of the reporting persons disclaims the existence of a Section 13(d) "group" as between the TRV III related parties and the TRV IV related parties and this report shall not be deemed an admission that any of such parties is or may be part of such a group with any of the other parties.

SEC source: 0000899243-20-019850 · 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

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Third Rock Ventures IV, L.P. · CIK 0001688327 · 10% owner

Third Rock Ventures GP IV, L.P. · CIK 0001688326 · 10% owner

TRV GP IV, LLC · CIK 0001688325 · 10% owner

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

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 9,970,454

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

F2 · ownership.direct: 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 the reporting persons, except for TRV III, 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.

F3 · ownership.direct: Each share of Series A Preferred Stock automatically converted into shares of the Issuer's common stock, par value $0.001 per share ("Common Stock") on a 1-for-3.55092 basis upon the closing of the Issuer's initial public offering on July 20, 2020 and had no expiration date.

F7 · ownership.direct: Each of the reporting persons disclaims the existence of a Section 13(d) "group" as between the TRV III related parties and the TRV IV related parties and this report shall not be deemed an admission that any of such parties is or may be part of such a group with any of the other parties.

SEC source: 0000899243-20-015874 · doc4.xml · derivative · entry 4

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

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Third Rock Ventures IV, L.P. · CIK 0001688327 · 10% owner

Third Rock Ventures GP IV, L.P. · CIK 0001688326 · 10% owner

TRV GP IV, LLC · CIK 0001688325 · 10% owner

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

10b5-1 filing: Not disclosed

Ownership: Indirect · See Footnotes

Disclosed balance after: 0

Underlying: Common Stock · 2,132,867 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 converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

F2 · derivative.exercise_date: The Series A Preferred Stock converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

F2 · derivative.expiration_date: The Series A Preferred Stock converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

F6 · ownership.nature: These shares are directly held by Third Rock Ventures IV.

F7 · ownership.nature: The general partner of TRV IV is Third Rock Ventures GP IV, L.P. ("TRV GP IV"). The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Each of TRV GP IV and TRV GP IV LLC disclaims beneficial ownership of these shares except to the extent of its 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.

F8 · ownership.nature: Each of Reporting Persons disclaims the existence of a Section 13(d) "group" as between the TRV III related parties and the TRV IV related parties and this report shall not be deemed an admission that any of such parties is or may be part of such a group with any of the other parties.

SEC source: 0000899243-20-015874 · doc4.xml · derivative · entry 3

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

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Third Rock Ventures IV, L.P. · CIK 0001688327 · 10% owner

Third Rock Ventures GP IV, L.P. · CIK 0001688326 · 10% owner

TRV GP IV, LLC · CIK 0001688325 · 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 · 5,559,440 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 converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

F2 · derivative.exercise_date: The Series A Preferred Stock converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

F2 · derivative.expiration_date: The Series A Preferred Stock converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

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

F4 · ownership.direct: 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 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.

F8 · ownership.direct: Each of Reporting Persons disclaims the existence of a Section 13(d) "group" as between the TRV III related parties and the TRV IV related parties and this report shall not be deemed an admission that any of such parties is or may be part of such a group with any of the other parties.

SEC source: 0000899243-20-015874 · doc4.xml · nonDerivative · entry 2

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

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Third Rock Ventures IV, L.P. · CIK 0001688327 · 10% owner

Third Rock Ventures GP IV, L.P. · CIK 0001688326 · 10% owner

TRV GP IV, LLC · CIK 0001688325 · 10% owner

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

10b5-1 filing: Not disclosed

Ownership: Indirect · See Footnotes

Disclosed balance after: 2,132,867

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.

F5 · shares: Represents the total number of shares of Common Stock received by Third Rock Ventures IV, L.P. ("TRV IV") upon conversion of the preferred stock of the Issuer in connection with the closing of the Issuer's initial public offering.

F2 · price: The Series A Preferred Stock converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

F6 · ownership.nature: These shares are directly held by Third Rock Ventures IV.

F7 · ownership.nature: The general partner of TRV IV is Third Rock Ventures GP IV, L.P. ("TRV GP IV"). The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Each of TRV GP IV and TRV GP IV LLC disclaims beneficial ownership of these shares except to the extent of its 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.

F8 · ownership.nature: Each of Reporting Persons disclaims the existence of a Section 13(d) "group" as between the TRV III related parties and the TRV IV related parties and this report shall not be deemed an admission that any of such parties is or may be part of such a group with any of the other parties.

SEC source: 0000899243-20-015874 · 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

STARR KEVIN P · CIK 0001192603 · 10% owner

TEPPER ROBERT I · CIK 0001192595 · 10% owner

Third Rock Ventures IV, L.P. · CIK 0001688327 · 10% owner

Third Rock Ventures GP IV, L.P. · CIK 0001688326 · 10% owner

TRV GP IV, LLC · CIK 0001688325 · 10% owner

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

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 5,839,160

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 III, L.P. ("TRV III") upon conversion of the preferred stock of the Issuer in connection with the closing of the Issuer's initial public offering.

F2 · price: The Series A Preferred Stock converted into Common Stock on a 7.15-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A Preferred Stock had no expiration date.

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

F4 · ownership.direct: 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 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.

F8 · ownership.direct: Each of Reporting Persons disclaims the existence of a Section 13(d) "group" as between the TRV III related parties and the TRV IV related parties and this report shall not be deemed an admission that any of such parties is or may be part of such a group with any of the other parties.

SEC source: 0000899243-20-004930 · doc4.xml · derivative · entry 6

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

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

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 · 1,909,317 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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.

F5 · 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 Levin, Starr and 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: 0000899243-20-004930 · doc4.xml · derivative · entry 5

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

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

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 · 49,879 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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 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 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: 0000899243-20-004930 · doc4.xml · derivative · entry 4

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

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

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 · 342,505 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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 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 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: 0000899243-20-004930 · doc4.xml · derivative · entry 3

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

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

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 · 8,220,641 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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 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 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: 0000899243-20-004930 · doc4.xml · nonDerivative · entry 2

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

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

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: 1,909,317

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.

F4 · 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 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, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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 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 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: 0000899243-20-004930 · 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

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

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: 9,024,031

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 III, L.P. ("TRV III") upon conversion of the Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock of the Issuer in connection with the closing of the Issuer's initial public offering.

F2 · price: The Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock converted into Common Stock on a one-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock, Series B Preferred Stock and Series C 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 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 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: 0000899243-19-020026 · doc4.xml · derivative · entry 4

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: 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: On July 22, 2019, the Series A Preferred Stock converted into Common Stock on a seven-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.

F2 · shares: As of the date of this filing, Third Rock Ventures IV, L.P. ("TRV IV") holds 2,261,904 shares of Common Stock issued upon conversion of 15,833,333 shares of Series A Preferred Stock. The general partner of TRV IV is Third Rock Ventures GP IV, L.P. ("TRV GP IV"). The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Abbie Celniker, Ph.D., Tepper, Alexis Borisy, Craig Muir and Cary Pfeffer, M.D. are the managing members of TRV IV LLC who collectively make voting and investment decisions with respect to shares held by TRV IV LP. Tepper disclaims beneficial ownership of the shares except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that he is the beneficial owner of such shares.

F3 · derivative.exercise_date: On July 22, 2019, the Series A Preferred Stock converted into Common Stock on a seven-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.

F3 · derivative.expiration_date: On July 22, 2019, the Series A Preferred Stock converted into Common Stock on a seven-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.

F2 · derivative.underlying_shares: As of the date of this filing, Third Rock Ventures IV, L.P. ("TRV IV") holds 2,261,904 shares of Common Stock issued upon conversion of 15,833,333 shares of Series A Preferred Stock. The general partner of TRV IV is Third Rock Ventures GP IV, L.P. ("TRV GP IV"). The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Abbie Celniker, Ph.D., Tepper, Alexis Borisy, Craig Muir and Cary Pfeffer, M.D. are the managing members of TRV IV LLC who collectively make voting and investment decisions with respect to shares held by TRV IV LP. Tepper disclaims beneficial ownership of the shares except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that he is the beneficial owner of such shares.

F1 · ownership.direct: The 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 LP"). The general partner of TRV GP III LP 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 LP, 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: 0000899243-19-020026 · doc4.xml · derivative · entry 3

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: 0

Underlying: Common Stock · 5,595,238 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: On July 22, 2019, the Series A Preferred Stock converted into Common Stock on a seven-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.

F3 · derivative.exercise_date: On July 22, 2019, the Series A Preferred Stock converted into Common Stock on a seven-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.

F3 · derivative.expiration_date: On July 22, 2019, the Series A Preferred Stock converted into Common Stock on a seven-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.

F1 · ownership.direct: The 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 LP"). The general partner of TRV GP III LP 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 LP, 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: 0000899243-19-020026 · doc4.xml · nonDerivative · entry 2

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: 5,962,202

F2 · balance_after: As of the date of this filing, Third Rock Ventures IV, L.P. ("TRV IV") holds 2,261,904 shares of Common Stock issued upon conversion of 15,833,333 shares of Series A Preferred Stock. The general partner of TRV IV is Third Rock Ventures GP IV, L.P. ("TRV GP IV"). The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Abbie Celniker, Ph.D., Tepper, Alexis Borisy, Craig Muir and Cary Pfeffer, M.D. are the managing members of TRV IV LLC who collectively make voting and investment decisions with respect to shares held by TRV IV LP. Tepper disclaims beneficial ownership of the shares except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that he is the beneficial owner of such shares.

F1 · ownership.direct: The 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 LP"). The general partner of TRV GP III LP 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 LP, 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: 0000899243-19-020026 · 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: 5,880,952

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 · price: On July 22, 2019, the Series A Preferred Stock converted into Common Stock on a seven-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial pubic offering. The shares had no expiration date.

F2 · balance_after: As of the date of this filing, Third Rock Ventures IV, L.P. ("TRV IV") holds 2,261,904 shares of Common Stock issued upon conversion of 15,833,333 shares of Series A Preferred Stock. The general partner of TRV IV is Third Rock Ventures GP IV, L.P. ("TRV GP IV"). The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Abbie Celniker, Ph.D., Tepper, Alexis Borisy, Craig Muir and Cary Pfeffer, M.D. are the managing members of TRV IV LLC who collectively make voting and investment decisions with respect to shares held by TRV IV LP. Tepper disclaims beneficial ownership of the shares except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that he is the beneficial owner of such shares.

F1 · ownership.direct: The 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 LP"). The general partner of TRV GP III LP 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 LP, 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: 0000899243-18-024777 · 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: 6,391,176

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 September 19, 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 September 19, 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: 0001104659-18-043853 · a4.xml · derivative · entry 4

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

LEVIN MARK J · CIK 0001180428 · 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 · 341,637 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: 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.

F2 · 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.

F2 · 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.

F2 · 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.

F4 · ownership.direct: 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: 0001104659-18-043853 · a4.xml · derivative · entry 3

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

LEVIN MARK J · CIK 0001180428 · 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 · 8,999,999 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: 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.

F2 · 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.

F2 · 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.

F2 · 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.

F4 · ownership.direct: 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: 0001104659-18-043853 · a4.xml · nonDerivative · entry 2

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

LEVIN MARK J · CIK 0001180428 · 10% owner

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

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 9,741,636

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 · shares: Represents the total number of shares of Common Stock received by TRV III upon the conversion of the Issuer's Series B Preferred Stock in connection with the closing of the Issuer's initial public offering.

F2 · 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 · ownership.direct: 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: 0001104659-18-043853 · a4.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

LEVIN MARK J · CIK 0001180428 · 10% owner

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

10b5-1 filing: Not disclosed

Ownership: Direct

Disclosed balance after: 9,399,999

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 III, L.P. ("TRV III") upon the conversion of the Issuer's Series A Preferred Stock in connection with the closing of the Issuer's initial public offering.

F2 · 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 · ownership.direct: 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: 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: 0000899243-17-010312 · 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: 0000899243-17-006623 · 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.

SEC source: 0000899243-17-006623 · 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

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: 6,475,191

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: 0000899243-17-002744 · doc4.xml · derivative · 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

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: 0

Underlying: Common Stock · 3,048,780 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: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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 · derivative.exercise_date: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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 · derivative.expiration_date: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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.

F5 · ownership.direct: 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 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.

SEC source: 0000899243-17-002744 · doc4.xml · derivative · 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

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: 0

Underlying: Common Stock · 9,688,344 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: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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 · derivative.exercise_date: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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 · derivative.expiration_date: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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.

F4 · 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: 0000899243-17-002744 · 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

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: 3,048,780

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

F3 · price: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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.

F5 · ownership.direct: 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 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.

SEC source: 0000899243-17-002744 · 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

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

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: 10,230,349

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 Series A Preferred Stock in connection with the closing of the Issuer's initial public offering.

F3 · price: Each share of Series A Preferred Stock converted into Common Stock on a one-for-3.69 basis upon the closing of the Issuer's initial public offering without payment or consideration. Each share of Series A Preferred Stock was 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.

F4 · 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: 0000899243-16-036195 · 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: 10,311,176

F1 · kind: On December 19, 2016, Third Rock Ventures III, L.P. ("TRV III") distributed, for no consideration, 750,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to Third Rock Ventures GP III, L.P. ("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: On December 19, 2016, Third Rock Ventures III, L.P. ("TRV III") distributed, for no consideration, 750,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to Third Rock Ventures GP III, L.P. ("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 ("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: 0000899243-16-033679 · 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: 1,285,713

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.

SEC source: 0000899243-16-033679 · 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

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: 9,475,191

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.