SEPN · Septerna, Inc.

RA Capital Management, L.P. (the "Adviser") is the investment manager for RA Capital Healthcare Fund, L.P. (the "Fund") and RA Capital Nexus Fund III, L.P. (the "Nexus Fund III"). The general partner of the Adviser is RA Capital Management GP, LLC (the "Adviser GP"), of which Dr. Peter Kolchinsky and Mr. Rajeev Shah are the managing members. Each of the Adviser, the Adviser GP, the Fund, the Nexus Fund III, Dr. Kolchinsky and Mr. Shah disclaims beneficial ownership of any of the reported securities, except to the extent of its or his respective pecuniary interest therein.

Held directly by the Nexus Fund III.

The reported transactions were effected pursuant to a Rule 10b5-1 trading plan dated March 26, 2026, previously adopted by the Reporting Person.

The price reported is a weighted average price. These shares were sold in multiple transactions at prices ranging from $50.05 to $50.81, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (2).

These shares are directly held by Third Rock Ventures V, L.P. ("TRV V"). The general partner of TRV V is Third Rock Ventures GP V, L.P. ("TRV GP V"). The general partner of TRV GP V is TRV GP V, LLC ("TRV GP V LLC"). Each of TRV GP V and TRV GP V LLC disclaims beneficial ownership of the shares except to the extent of its pecuniary interest therein, if any, and this report shall not be deemed an admission that it is the beneficial owner of such shares.

The reported transactions were effected pursuant to a Rule 10b5-1 trading plan dated March 26, 2026, previously adopted by the Reporting Person.

The price reported is a weighted average price. These shares were sold in multiple transactions at prices ranging from $51.00 to $51.10, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (4).

These shares are directly held by Third Rock Ventures V, L.P. ("TRV V"). The general partner of TRV V is Third Rock Ventures GP V, L.P. ("TRV GP V"). The general partner of TRV GP V is TRV GP V, LLC ("TRV GP V LLC"). Each of TRV GP V and TRV GP V LLC disclaims beneficial ownership of the shares except to the extent of its pecuniary interest therein, if any, and this report shall not be deemed an admission that it is the beneficial owner of such shares.

RA Capital Management, L.P. (the "Adviser") is the investment manager for RA Capital Healthcare Fund, L.P. (the "Fund") and RA Capital Nexus Fund III, L.P. (the "Nexus Fund III"). The general partner of the Adviser is RA Capital Management GP, LLC (the "Adviser GP"), of which Dr. Peter Kolchinsky and Mr. Rajeev Shah are the managing members. Each of the Adviser, the Adviser GP, the Fund, the Nexus Fund III, Dr. Kolchinsky and Mr. Shah disclaims beneficial ownership of any of the reported securities, except to the extent of its or his respective pecuniary interest therein.

Held directly by the Fund.

1/48th of the shares subject to such option vest and become exercisable in substantially equal monthly installments on each monthly anniversary of October 28, 2024, subject to the Reporting Person's continuous service to the Issuer on each such date.

1/48th of the shares subject to such option vest and become exercisable in substantially equal monthly installments on each monthly anniversary of August 1, 2023, subject to the Reporting Person's continuous service to the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on October 20, 2025.

1/48th of the shares subject to such option shall vest and become exercisable in substantially equal monthly installments on each monthly anniversary of March 1, 2024, subject to the Reporting Person's continuous service to the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on March 20, 2026.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $34.43 to $35.04. 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 range.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on March 20, 2026.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $33.39 to $34.32. 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 range.

1/48th of the shares subject to such option shall vest and become exercisable in substantially equal monthly installments on each monthly anniversary of August 1, 2023, subject to the Reporting Person's continuous service to the Issuer on each such date.

1/48th of the shares subject to such option vest and become exercisable in substantially equal monthly installments on each monthly anniversary of October 28, 2024, subject to the Reporting Person's continuous service to the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on October 20, 2025.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $33.49 to $34.24. 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 range.

1/48th of the shares subject to such option vest and become exercisable in substantially equal monthly installments on each monthly anniversary of August 1, 2023, subject to the Reporting Person's continuous service to the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on March 20, 2026.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $34.52 to $35.10. 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 range.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on March 20, 2026.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $33.49 to $34.46. 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 range.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on October 31, 2025.

1/48th of the shares subject to such option vest and become exercisable in substantially equal monthly installments on each monthly anniversary of October 28, 2024, subject to the Reporting Person's continuous service to the Issuer on each such date.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to the Reporting Person's continued service to the Issuer through such vesting date.

Under the Reporting Person's arrangement with RA Capital Management, L.P. (the "Adviser"), the Reporting Person holds the option for the benefit of the R.A. Capital Healthcare Fund, L.P. (the "Fund") and R.A. Capital Nexus Fund III, L.P. (the "Nexus Fund III"). The Reporting Person is obligated to turn over to the Adviser any net cash or stock received upon exercise of the option, which will offset advisory fees owed by the Fund and the Nexus Fund III to the Adviser. The Reporting Person therefore disclaims beneficial ownership of the option and underlying common stock.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to the Reporting Person's continued service to the Issuer through such vesting date.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to the Reporting Person's continued service to the Issuer through such vesting date.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to the Reporting Person's continued service to the Issuer through such vesting date.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to Dr. Simson's continued service to the Issuer through such vesting date.

RA Capital Management, L.P. (the "Adviser") is the investment manager for RA Capital Healthcare Fund, L.P. (the "Fund") and RA Capital Nexus Fund III, L.P. (the "Nexus Fund III"). The general partner of the Adviser is RA Capital Management GP, LLC (the "Adviser GP"), of which Dr. Peter Kolchinsky and Mr. Rajeev Shah are the managing members. Each of the Adviser, the Adviser GP, the Fund, the Nexus Fund III, Dr. Kolchinsky and Mr. Shah disclaims beneficial ownership of any of the reported securities, except to the extent of its or his respective pecuniary interest therein.

Under Dr. Simson's arrangement with the Adviser, Dr. Simson holds the option for the benefit of the Fund and the Nexus Fund III. Dr. Simson is obligated to turn over to the Adviser any net cash or stock received upon exercise of the option, which will offset advisory fees owed by the Fund and the Nexus Fund III to the Adviser. The Reporting Persons therefore disclaim beneficial ownership of the option and underlying common stock.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to the Reporting Person's continued service to the Issuer through such vesting date.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to the Reporting Person's continued service to the Issuer through such vesting date.

The shares underlying the stock option vest in full upon the earlier of (i) June 26, 2027 or (ii) the date of the Issuer's next annual meeting of stockholders, subject to the Reporting Person's continued service to the Issuer through such vesting date.

Includes 3,501 shares acquired under the Septerna, Inc. 2024 Employee Stock Purchase Plan on April 30, 2026 in a transaction that was exempt under Rule 16b-3(c).

1/48th of the shares subject to such option vest and become exercisable in substantially equal monthly installments on each monthly anniversary of August 1, 2023, subject to the Reporting Person's continuous service to the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on March 20, 2026.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $34.70 to $35.64. 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 range.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on March 20, 2026.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $35.71 to $36.34. 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 range.

1/48th of the shares subject to such option shall vest and become exercisable in substantially equal monthly installments on each monthly anniversary of August 1, 2025, subject to the Reporting Person's continuous service to the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on October 31, 2025.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $35.00 to $35.19. 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 range.

1/48th of the shares subject to such option shall vest and become exercisable in substantially equal monthly installments on each monthly anniversary of August 1, 2025, subject to the Reporting Person's continuous service to the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on October 31, 2025.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $27.24 to $28.23. 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 range.

25% of the shares underlying this option vested on February 1, 2024, with the remainder vesting in thirty-six substantially equal monthly installments thereafter, subject to the Reporting Person's continuous service the Issuer on each such date.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on October 31, 2025.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $28.25 to $29.15. 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 range.

This transaction was effected pursuant to a Rule 10b5-1 sales plan adopted by the Reporting Person on October 31, 2025.

The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $29.29 to $30.08. 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 range.

25% of the shares underlying this option vested on February 1, 2024, with the remainder vesting in thirty-six substantially equal monthly installments thereafter, subject to the Reporting Person's continuous service the Issuer on each such date.