Amylyx Pharmaceuticals, Inc.

1. Represents shares required to be sold by the Reporting Person to cover tax withholding obligations in connection with the vesting of restricted stock units. Such sales were automatic and not at the discretion of the Reporting Person.

2. The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $14.450 to $15.385. Full information regarding the number of shares sold at each separate price can be furnished to the SEC staff upon request.

3. Shares held by the JAMES M. FRATES 2025 GRAT NO 1 U/A DTD 12/12/2025, for which the Reporting Person serves as trustee and is the sole annuitant. The Reporting Person disclaims beneficial ownership of such shares for purposes of Section 16 except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that such shares are beneficially owned by him for Section 16 or any other purpose.

4. Shares held by the James M. Frates 2024 Grantor Retained Annuity Trust No. 1, for which the Reporting Person serves as trustee and is the sole annuitant. The Reporting Person disclaims beneficial ownership of such shares for purposes of Section 16 except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that such shares are beneficially owned by him for Section 16 or any other purpose.

5. Shares held by the FRATES FAMILY 2013 IRREV TRUST, for which the Reporting Person serves as trustee. The Reporting Person disclaims beneficial ownership of such shares for purposes of Section 16 except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that such shares are beneficially owned by him for Section 16 or any other purpose.