ALNYLAM PHARMACEUTICALS, INC.

1. These shares are represented by restricted stock units (RSUs) granted under the Second Amended and Restated 2018 Stock Incentive Plan. Each RSU represents a contingent right to receive one share of common stock. The RSUs will vest over a three-year period, with one third vesting on each of the first, second and third anniversaries of the grant date, subject to the Reporting Persons continuous service with the Issuer as of each such vesting date.

2. Represents shares automatically sold by the Company on behalf of the reporting person pursuant to a mandatory sell-to-cover provision in the award agreement to cover minimum statutory tax withholding obligations.

3. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $323.16 to $324.15. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

4. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $324.20 to $325.20. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

5. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $325.21 to $326.21. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

6. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $326.23 to $327.23. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

7. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $327.24 to $328.24. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

8. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $328.28 to $329.26. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

9. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $329.29 to $330.29. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

10. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $330.30 to $331.30. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

11. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $331.32 to $332.31. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

12. The price reported represents the weighted average sales price of shares sold in multiple transactions at prices ranging from $332.34 to $332.24. The reporting person will provide to the issuer, any security holder of the issuer or the SEC staff, upon request, information regarding the number of shares sold at each price within the range.

13. Reflects shares of ALNY common stock acquired by the Reporting Person under the issuer 401(k) plan as a result of the issuer 401(k) matching contribution program.

14. The stock option vests as to 25% of the shares on the first anniversary of the grant date and the remaining shares vest in equal installments at the end of each successive three-month period thereafter until the fourth anniversary of the grant date, subject to the Reporting Persons continuous service with the Issuer as of each such vesting date.

15. Stock price performance unit award (PSU) granted pursuant to the Second Amended and Restated 2018 Stock Incentive Plan. The amount reported is the maximum number of PSUs that may be earned upon achievement of pre-set stock price thresholds. Subject to the terms and conditions of the PSU award agreement, the PSU vests on December 31, 2029 (Vesting Date) and between 50% and 200% of the target number of 55,373 PSUs may be earned if the highest average closing price that is achieved over a 30 consecutive trading period (Future Average Stock Price) in the six months preceding the vesting date (Measurement Period) meets or exceeds certain thresholds. If the Future Average Stock Price does not meet or exceed the baseline threshold price during the Measurement Period, no portion of the PSU will be earned, and all PSUs will be forfeited on the Vesting Date.