Item 1.01. Entry into a Material Definitive Agreement.

On September 20, 2012, Alnylam Pharmaceuticals, Inc. (the "Company") and Novartis Pharma AG ("Novartis") entered into a letter agreement (the "Letter Agreement"), pursuant to which the Company has agreed that, if during the next five years Novartis is not eligible to sell pursuant to Rule 144(b)(1) shares of the Company's common stock it holds, excluding ineligibility resulting from the purchase by Novartis or its affiliates of additional shares of the Company's common stock, Novartis will have registration rights for such shares similar to those rights set forth in the Investor Rights Agreement dated September 6, 2005 by and between the Company and Novartis (the "Investor Rights Agreement"). The Letter Agreement was executed concurrent with the sale by Novartis of 1,551,896 shares of common stock of the Company which the Company expects will result in the loss of Novartis' existing demand and piggyback registration rights, as well as its subscription rights, under the Investor Rights Agreement. Following this sale, Novartis reported that it holds 4,051,002 shares of the Company's common stock.

---