The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

NAMES OF REPORTING PERSONS

Third Rock Ventures IV, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

(a)  ☐        (b)  ☒

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

    Delaware

NUMBER OF  

SHARES   BENEFICIALLY   OWNED BY  

EACH  

REPORTING  

PERSON  

WITH  

SOLE VOTING POWER

0

SHARED VOTING POWER

8,008,204

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

8,008,204

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

8,008,204

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

24.2% (1)

TYPE OF REPORTING PERSON

PN

The percent of class was calculated based on 33,151,539 shares of common stock issued and outstanding as of October 31, 2018, as disclosed in the Issuer’s Form 10-Q filed with the Securities and Exchange Commission on November 8, 2018.

NAMES OF REPORTING PERSONS

Third Rock Ventures GP IV, L.P.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

(a)  ☐        (b)  ☒

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

    Delaware

NUMBER OF  

SHARES   BENEFICIALLY   OWNED BY  

EACH  

REPORTING  

PERSON  

WITH  

SOLE VOTING POWER

0

SHARED VOTING POWER

8,008,204

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

8,008,204

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

8,008,204

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

24.2% (1)

TYPE OF REPORTING PERSON

PN

NAMES OF REPORTING PERSONS

TRV GP IV, LLC

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

(a)  ☐        (b)  ☒

SEC USE ONLY

CITIZENSHIP OR PLACE OF ORGANIZATION

    Delaware

NUMBER OF  

SHARES   BENEFICIALLY   OWNED BY  

EACH  

REPORTING  

PERSON  

WITH  

SOLE VOTING POWER

0

SHARED VOTING POWER

8,008,204

SOLE DISPOSITIVE POWER

0

SHARED DISPOSITIVE POWER

8,008,204

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

8,008,204

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

24.2% (1)

TYPE OF REPORTING PERSON

OO

100 Technology Square

Cambridge, Massachusetts

(i) Third Rock Ventures IV, L.P. (“TRV IV”);

(ii)  Third Rock Ventures GP IV, L.P. (“TRV GP IV”), which is the sole general partner of TRV IV; and

(iii)  TRV GP IV, LLC (“TRV GP IV LLC”), which is the sole general partner of TRV GP IV.

The address of the principal business office of each of the Reporting Persons is Third Rock Ventures, LLC, 29 Newbury Street, 3rd Floor, Boston, MA 02116.

Each of TRV IV and TRV GP IV is a Delaware limited partnership. TRV GP IV LLC is a Delaware limited liability company.

CUSIP Number:

55910K108

Broker or dealer registered under Section 15 of the Act;

Bank as defined in Section 3(a)(6) of the Act;

Insurance company as defined in Section 3(a)(19) of the Act;

Investment company registered under Section 8 of the Investment Company Act of 1940;

An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);

An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);

A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940;

A non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J);

Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K). If filing as a non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J), please specify the type of institution:

If filing as a non-U.S. institution in accordance with §240.13d-1(b)(1)(ii)(J), please specify the type of institution:

Ownership.

TRV IV directly owns 8,008,204 shares of Common Stock (the “Shares”), which represents approximately 24.2% of the outstanding shares of Common Stock.

TRV GP IV is the general partner of TRV IV and may be deemed to beneficially own the Shares.

TRV GP IV LLC is the general partner of TRV GP IV and may be deemed to beneficially own the Shares.

Number of shares as to which such person has:

Reporting Person

TRV IV

TRV GP IV

TRV GP IV LLC

Sole power to vote or direct the vote

Shared power to vote or to direct the vote

Sole power to dispose or to direct the disposition of

Shared power to dispose or to direct the disposition of

Ownership of Five Percent or Less of a Class.

Ownership of More than Five Percent on Behalf of Another Person.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.

Identification and Classification of Members of the Group.

Notice of Dissolution of Group.

Certification.