SLS · SELLAS Life Sciences Group, Inc.

The Reporting Person received an award of 1,065,000 restricted stock units on June 16, 2026, which shall vest as follows: (i) 25% on December 1, 2026 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The Reporting Person received an award of 50,000 restricted stock units on January 7, 2026, which shall vest in full on December 1, 2026, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 50,000 restricted stock units on January 7, 2026, which shall vest in full on December 1, 2026, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 300,000 restricted stock units on January 7, 2026, which shall vest as follows: (i) 25% on December 1, 2026 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The Reporting Person received an award of 300,000 restricted stock units on January 7, 2026, which shall vest as follows: (i) 25% on December 1, 2026 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The Reporting Person received an award of 50,000 restricted stock units on January 7, 2026, which shall vest in full on December 1, 2026, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 50,000 restricted stock units on January 7, 2026, which shall vest in full on December 1, 2026, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 50,000 restricted stock units on January 7, 2026, which shall vest in full on December 1, 2026, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

Transaction relates to shares of common stock withheld by the SELLAS Life Sciences Group, Inc. (the "Issuer") to cover tax withholding obligations of the Reporting Person in connection with the vesting of Restricted Stock Units ("RSUs").

Represents the number shares of common stock withheld by the Issuer to cover tax withholding obligations of the Reporting Person in connection with the vesting of RSUs.

Transaction relates to shares of common stock withheld by the SELLAS Life Sciences Group, Inc. (the "Issuer") to cover tax withholding obligations of the Reporting Person in connection with the vesting of Restricted Stock Units ("RSUs").

Represents the number shares of common stock withheld by the Issuer to cover tax withholding obligations of the Reporting Person in connection with the vesting of RSUs.

Transaction relates to shares of common stock withheld by the SELLAS Life Sciences Group, Inc. (the "Issuer") to cover tax withholding obligations of the Reporting Person in connection with the vesting of Restricted Stock Units ("RSUs").

Represents the number shares of common stock withheld by the Issuer to cover tax withholding obligations of the Reporting Person in connection with the vesting of RSUs.

The price reflects the weighted average purchase price of the shares of common stock purchased. These shares of common stock were purchased in multiple transactions at prices ranging from $1.555 to $1.625 per share. Ms. Kalin undertakes to provide, upon request by the staff of the Securities and Exchange Commission, the Issuer or any security holder of the Issuer, full information regarding the number of shares of common stock purchased at each price.

The shares were purchased in multiple transactions at a price of $1.69. The reporting person undertakes to provide SELLAS Life Sciences Group, Inc., any security holder of SELLAS Life Sciences Group, Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at the price set forth in this footnote.

The Reporting Person received an award of 12,500 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

One-quarter (1/4) of the option shares will vest and became exercisable one year from the January 10, 2025 grant date, and the remainder will vest and become exercisable in equal monthly installments for thirty-six (36) months thereafter, provided that the Reporting Person remains in service with the Issuer through each such monthly vesting date.

The Reporting Person received an award of 257,000 restricted stock units on January 10, 2025, which shall vest as follows: (i) 25% on December 1, 2025 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The Reporting Person received an award of 243,900 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The shares subject to the option will fully vest on the earlier of (x) January 10, 2026 (the first anniversary of the date of grant) or (y) the business day prior to the 2026 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

One-quarter (1/4) of the option shares will vest and became exercisable one year from the January 10, 2025 grant date, and the remainder will vest and become exercisable in equal monthly installments for thirty-six (36) months thereafter, provided that the Reporting Person remains in service with the Issuer through each such monthly vesting date.

The Reporting Person received an award of 113,000 restricted stock units on January 10, 2025, which shall vest as follows: (i) 25% on December 1, 2025 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The Reporting Person received an award of 12,500 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 81,300 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The shares subject to the option will fully vest on the earlier of (x) January 10, 2026 (the first anniversary of the date of grant) or (y) the business day prior to the 2026 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The shares subject to the option will fully vest on the earlier of (x) January 10, 2026 (the first anniversary of the date of grant) or (y) the business day prior to the 2026 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 113,000 restricted stock units on January 10, 2025, which shall vest as follows: (i) 25% on December 1, 2025 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The Reporting Person received an award of 12,500 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 12,500 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 81,300 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

One-quarter (1/4) of the option shares will vest and became exercisable one year from the January 10, 2025 grant date, and the remainder will vest and become exercisable in equal monthly installments for thirty-six (36) months thereafter, provided that the Reporting Person remains in service with the Issuer through each such monthly vesting date.

The Reporting Person received an award of 12,500 restricted stock units on January 10, 2025, which shall vest in full on December 1, 2025, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The shares subject to the option will fully vest on the earlier of (x) January 10, 2026 (the first anniversary of the date of grant) or (y) the business day prior to the 2026 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The shares subject to the option will fully vest on the earlier of (x) January 10, 2026 (the first anniversary of the date of grant) or (y) the business day prior to the 2026 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

Transaction relates to shares of common stock withheld by the SELLAS Life Sciences Group, Inc. (the "Issuer") to cover tax withholding obligations of the Reporting Person in connection with the vesting of Restricted Stock Units ("RSUs").

Represents the number shares of common stock withheld by the Issuer to cover tax withholding obligations of the Reporting Person in connection with the vesting of RSUs.

Transaction relates to shares of common stock withheld by the SELLAS Life Sciences Group, Inc. (the "Issuer") to cover tax withholding obligations of the Reporting Person in connection with the vesting of Restricted Stock Units ("RSUs").

Represents the number shares of common stock withheld by the Issuer to cover tax withholding obligations of the Reporting Person in connection with the vesting of RSUs.

Transaction relates to shares of common stock withheld by the SELLAS Life Sciences Group, Inc. (the "Issuer") to cover tax withholding obligations of the Reporting Person in connection with the vesting of Restricted Stock Units ("RSUs").

Represents the number shares of common stock withheld by the Issuer to cover tax withholding obligations of the Reporting Person in connection with the vesting of RSUs.

One-quarter (1/4) of the option shares will vest and became exercisable one year from the January 22, 2024 grant date, and the remainder will vest and become exercisable in equal monthly installments for thirty-six (36) months thereafter, provided that the Reporting Person remains in service with the Issuer through each such monthly vesting date.

The shares subject to the option will fully vest on the earlier of (x) January 22, 2025 (the first anniversary of the date of grant) or (y) the business day prior to the 2025 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 6,000 restricted stock units on January 22, 2024, which shall vest in full on December 1, 2024, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 35,000 restricted stock units on January 22, 2024, which shall vest as follows: (i) 25% on December 1, 2024 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The Reporting Person received an award of 6,000 restricted stock units on January 22, 2024, which shall vest in full on December 1, 2024, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 20,000 restricted stock units on January 22, 2024, which shall vest as follows: (i) 25% on December 1, 2024 and (ii) 25% on each December 1 thereafter until fully vested, provided that the Reporting Person remains in service with the Issuer through each such vesting date.

The shares subject to the option will fully vest on the earlier of (x) January 22, 2025 (the first anniversary of the date of grant) or (y) the business day prior to the 2025 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

One-quarter (1/4) of the option shares will vest and became exercisable one year from the January 22, 2024 grant date, and the remainder will vest and become exercisable in equal monthly installments for thirty-six (36) months thereafter, provided that the Reporting Person remains in service with the Issuer through each such monthly vesting date.

The Reporting Person received an award of 6,000 restricted stock units on January 22, 2024, which shall vest in full on December 1, 2024, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The shares subject to the option will fully vest on the earlier of (x) January 22, 2025 (the first anniversary of the date of grant) or (y) the business day prior to the 2025 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The shares subject to the option will fully vest on the earlier of (x) January 22, 2025 (the first anniversary of the date of grant) or (y) the business day prior to the 2025 annual stockholders meeting, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

The Reporting Person received an award of 6,000 restricted stock units on January 22, 2024, which shall vest in full on December 1, 2024, subject to the Reporting Person's continued service on the Issuer's Board of Directors through such date.

One-quarter (1/4) of the option shares will vest and became exercisable one year from the January 22, 2024 grant date, and the remainder will vest and become exercisable in equal monthly installments for thirty-six (36) months thereafter, provided that the Reporting Person remains in service with the Issuer through each such monthly vesting date.