X0306 4 2017-05-16 0 0001178670 ALNYLAM PHARMACEUTICALS, INC. ALNY 0001514291 Mason Michael C/O ALNYLAM PHARMACEUTICALS, INC. 300 THIRD STREET CAMBRIDGE MA 02142 0 1 0 0 VP, Finance and Treasurer Common Stock 2017-05-16 4 M 0 5625 18.66 A 5625 D Common Stock 2017-05-16 4 M 0 3750 7.1 A 9375 D Common Stock 2017-05-16 4 S 0 9375 70 D 0 D Common Stock 2176 I By Managed Account Stock Option (Right to Buy) 7.1 2017-05-16 4 M 0 3750 0 D 2021-11-30 Common Stock 3750 0 D Stock Option (Right to Buy) 18.66 2017-05-16 4 M 0 5625 0 D 2022-12-20 Common Stock 5625 0 D All sales reported on this Form 4 were made pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on August 9, 2016. The reporting person owns 2,176 shares of ALNY common stock under the ALNY 401(k) plan as a result of the ALNY 401(k) matching contribution program. The stock option vests as to 25% of the shares on the first anniversary of the date of the stock option grant and as to an additional 6.25% of the shares each successive three month period thereafter. /s/ Michael P. Mason 2017-05-17