SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

Septerna, Inc.

(Name of Issuer)

Common Stock, $0.001 par value per share

(Title of Class of Securities)

81734D104

(CUSIP Number)

12/31/2024

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13G

CUSIP No.

81734D104

Names of Reporting Persons

Third Rock Ventures V, L.P.

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 6,215,591.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 6,215,591.00

Aggregate Amount Beneficially Owned by Each Reporting Person

6,215,591.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

14.0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13G

CUSIP No.

81734D104

Names of Reporting Persons

Third Rock Ventures GP V, L.P.

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 6,215,591.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 6,215,591.00

Aggregate Amount Beneficially Owned by Each Reporting Person

6,215,591.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

14.0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13G

CUSIP No.

81734D104

Names of Reporting Persons

TRV GP V, LLC

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 6,215,591.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 6,215,591.00

Aggregate Amount Beneficially Owned by Each Reporting Person

6,215,591.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

14.0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13G

CUSIP No.

81734D104

Names of Reporting Persons

Third Rock Ventures VI, L.P.

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 3,152,456.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 3,152,456.00

Aggregate Amount Beneficially Owned by Each Reporting Person

3,152,456.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

7.1 %

Type of Reporting Person (See Instructions)

SCHEDULE 13G

CUSIP No.

81734D104

Names of Reporting Persons

Third Rock Ventures GP VI, L.P.

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 3,152,456.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 3,152,456.00

Aggregate Amount Beneficially Owned by Each Reporting Person

3,152,456.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

7.1 %

Type of Reporting Person (See Instructions)

SCHEDULE 13G

CUSIP No.

81734D104

Names of Reporting Persons

TRV GP VI, LLC

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 3,152,456.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 3,152,456.00

Aggregate Amount Beneficially Owned by Each Reporting Person

3,152,456.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

7.1 %

Type of Reporting Person (See Instructions)

SCHEDULE 13G

Item 1.

(a)

Name of issuer:

Septerna, Inc.

(b)

Address of issuer's principal executive offices:

250 East Grand AvenueSouth San Francisco, CA 94080

Item 2.

(a)

Name of person filing:

(i) Third Rock Ventures V, L.P. ("TRV V"); (ii) Third Rock Ventures GP V, L.P. ("TRV GP V"), which is the sole general partner of TRV V; (iii) TRV GP V, LLC ("TRV GP V LLC"), which is the sole general partner of TRV GP V; (iv) Third Rock Ventures VI, L.P. ("TRV VI"); (v) Third Rock Ventures GP VI, L.P. ("TRV GP VI"), which is the sole general partner of TRV VI; and (vi) TRV GP VI, LLC ("TRV GP VI LLC"), which is the sole general partner of TRV GP VI.

(b)

Address or principal business office or, if none, residence:

The address of the principal business office of each of the Reporting Persons is Third Rock Ventures, LLC, 201 Brookline Ave, Suite 1401, Boston, MA 02215.

(c)

Citizenship:

Each of TRV V, TRV GP V, TRV VI and TRV GP VI is a Delaware limited partnership.Each of TRV GP V LLC and TRV GP VI LLC is a Delaware limited liability company.

(d)

Title of class of securities:

Common Stock, $0.001 par value per share

(e)

CUSIP No.:

81734D104

Item 3.

If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);

(b)

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);

(c)

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);

(d)

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);

(e)

An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);

(f)

An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);

(g)

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

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

(i)

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 (15 U.S.C. 80a-3);

(j)

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

(k)

Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).

Item 4.

Ownership

(a)

Amount beneficially owned:

(i) TRV V directly owns 6,215,591 shares of Common Stock (the "V Shares") (ii) TRV GP V is the general partner of TRV V and may be deemed to beneficially own the V Shares. (iii) TRV GP V LLC is the general partner of TRV GP V and may be deemed to beneficially own the V Shares. (iv) TRV VI directly owns 3,152,456 shares of Common Stock (the "VI Shares"). (v) TRV GP VI is the general partner of TRV VI and may be deemed to beneficially own the VI Shares. (vi) TRV GP VI LLC is the general partner of TRV GP VI and may be deemed to beneficially own the VI Shares. Each of TRV V, TRV GP V, and TRV GP V LLC, on the one hand, and TRV VI, TRV GP VI and TRV GP VI LLC, on the other hand, disclaim the existence of a "group" for purposes of Rule 13d-5 of the Securities Exchange Act of 1934, as amended, and nothing contained in this report shall be deemed an admission that any such group exists or may exist.

(b)

Percent of class:

The V shares represent approximately 14.0% of the outstanding shares of Common Stock.The VI shares represent approximately 7.1% of the outstanding shares of Common Stock.The percent of class was calculated based on 44,402,742 shares of common stock issued and outstanding as of November 11, 2024, as disclosed in the Issuer's 10-Q filed with the Securities and Exchange Commission on November 20, 2024.

(c)

Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

TRV V - 0 TRV GP V - 0 TRV GP V LLC - 0 TRV VI - 0 TRV GP VI - 0 TRV GP VI LLC - 0

(ii) Shared power to vote or to direct the vote:

TRV V - 6,215,591 shares TRV GP V - 6,215,591 shares TRV GP V LLC - 6,215,591 shares TRV VI - 3,152,456 shares TRV GP VI - 3,152,456 shares TRV GP VI LLC - 3,152,456 shares

(iii) Sole power to dispose or to direct the disposition of:

TRV V - 0 TRV GP V - 0 TRV GP V LLC - 0 TRV VI - 0 TRV GP VI - 0 TRV GP VI LLC - 0

(iv) Shared power to dispose or to direct the disposition of:

TRV V - 6,215,591 shares TRV GP V - 6,215,591 shares TRV GP V LLC - 6,215,591 shares TRV VI - 3,152,456 shares TRV GP VI - 3,152,456 shares TRV GP VI LLC - 3,152,456 shares

Item 5.

Ownership of 5 Percent or Less of a Class.

Item 6.

Ownership of more than 5 Percent on Behalf of Another Person.

Not Applicable

Item 7.

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

Not Applicable

Item 8.

Identification and Classification of Members of the Group.

Not Applicable

Item 9.

Notice of Dissolution of Group.

Not Applicable

Item 10.

Certifications:

Not Applicable

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Third Rock Ventures V, L.P.

Signature:	/s/ Kevin Gillis
Name/Title:	Kevin Gillis,COO of TRV GP V, LLC, general partner of Third Rock Ventures GP V, L.P., general partner of Third Rock Ventures V, L.P.
Date:	02/13/2025

Third Rock Ventures GP V, L.P.

Signature:	/s/ Kevin Gillis
Name/Title:	Kevin Gillis, COO of TRV GP VI, LLC, general partner of Third Rock Ventures GP VI, L.P
Date:	02/13/2025

TRV GP V, LLC

Signature:	/s/ Kevin Gillis
Name/Title:	Kevin Gillis, Chief Operating Officer of TRV GP V, LLC
Date:	02/13/2025

Third Rock Ventures VI, L.P.

Signature:	/s/ Kevin Gillis
Name/Title:	Kevin Gillis, COO of TRV GP VI, LLC, general partner of Third Rock Ventures GP VI, L.P., general partner of Third Rock Ventures VI, L.P.
Date:	02/13/2025

Third Rock Ventures GP VI, L.P.

Signature:	/s/ Kevin Gillis
Name/Title:	Kevin Gillis, Chief Operating Officer of TRV GP VI, LLC, general partner of Third Rock Ventures GP VI, L.P.
Date:	02/13/2025

TRV GP VI, LLC

Signature:	/s/ Kevin Gillis
Name/Title:	Kevin Gillis, Chief Operating Officer of TRV GP VI, LLC
Date:	02/13/2025