MERRIMACK PHARMACEUTICALS, INC. 401(K) RETIREMENT PLAN

Profit-sharing plan.

This plan or any part of it is intended to meet the conditions of 29 CFR 2550.404c-1 (shifting investment responsibility to participants).

Participants have the opportunity to direct the investment of all of the assets allocated to their individual accounts regardless of whether 29 CFR 2550.404c-1 is intended to be met.

Cash or deferred arrangement described in Code section 401(k) that is part of a qualified defined contribution plan under which employees may elect to defer part of their compensation or receive these amounts in cash.

Employee contributions are allocated to separate accounts under the plan or employer contributions are based in whole or in part on employee deferrals or contributions to the plan. Not applicable to 401(k) plans with only QNECs and/or QMACs or to 403(b)(1) 403(b)(7) or 408 arrangements.

Total or partial participant-directed account plan where plan uses a default investment account for participants who fail to direct assets in their account.

A master prototype or volume submitter plan that is the subject of a favorable opinion or advisory letter from the IRS.