Alnylam Presents Pre-clinical Results from Hypercholesterolemia, Huntington's Disease and Neuropathic Pain Programs at Keystone Symposium on RNAi.

-- New Results Expand In Vivo Efficacy Data for Direct and Systemic RNAi Therapeutic Applications --

CAMBRIDGE, Mass. -- Alnylam Pharmaceuticals, Inc. (Nasdaq: ALNY), a leading RNAi therapeutics company, announced today that it presented new pre-clinical data at the "RNAi for Target Validation and as a Therapeutic" Keystone Symposium held January 28 -- February 2, 2007 in Keystone, Colorado. The new results were presented from Alnylam's RNAi therapeutics program targeting PCSK9 for the treatment of hypercholesterolemia and from Alnylam collaborations in Huntington's disease and neuropathic pain. The data demonstrate that small interfering RNAs (siRNAs), the molecules that mediate RNAi, can be administered in animal models to achieve therapeutic silencing of disease-causing genes, and that effective and clinically relevant delivery can be achieved with both direct and systemic RNAi applications.

"Alnylam scientists and our collaborators continue to make significant progress in achieving delivery for RNAi therapeutics and in demonstrating in vivo efficacy in animal models of human disease. Indeed, the new data presented this week further extend Alnylam's scientific leadership in advancing RNAi therapeutics as a new class of innovative medicines," said Victor Kotelianski, M.D., Ph.D., Vice President, Research at Alnylam. "We are especially encouraged by the new findings in our PCSK9 program for hypercholesterolemia that show considerable potency and durability in a mouse model after a single dose of our RNAi therapeutic. These new data support ongoing efforts to advance our PCSK9 program toward a planned IND filing in 2007."

Hypercholesterolemia

Alnylam is developing a systemically delivered RNAi therapeutic for the treatment of hypercholesterolemia targeting PCSK9, a key gene involved in the metabolism of LDL cholesterol. In a talk titled "Achieving Therapeutic Gene Silencing in vivo with RNAi," results were presented on ALN-PCS01, Alnylam's RNAi therapeutic targeting PCSK9 which comprises an optimized siRNA formulated in a liposomal nanoparticle.

Data presented include the following.

* In vivo systemic administration of ALN-PCS01 in mice was associated with dose-dependent and rapid silencing of the PCSK9 messenger RNA to more than 70 percent of control levels, with peak silencing effects observed as soon as 48 hours after dosing.

* After a single intravenous injection, the RNAi therapeutic showed a durable biological effect with more than 50 percent silencing of PCSK9 maintained through two weeks, and full recovery of PCSK9 to normal levels at 23 days after dosing.

* Therapeutic efficacy for ALN-PCS01 was demonstrated with up to 30 percent reduction in total cholesterol levels at doses that were well tolerated.

Alnylam intends to file an investigational new drug (IND) application for ALN-PCS01 in 2007.

Huntington's Disease

In a talk titled "Advances in the use of Synthetic siRNAs in the Treatment of Huntington's Disease Models," Alnylam collaborator Dr. Neil Aronin, Professor of Medicine and Cell Biology and Director of Endocrinology and Metabolism at the University of Massachusetts Medical School presented in vivo data demonstrating that an siRNA targeting the huntingtin gene inhibited the progression of Huntington's disease in a mouse model. These results showed both a reduction of neuronal pathology and an improvement in abnormal behavior in the disease model with administration of a cholesterol-conjugated siRNA. Pathological protein aggregates in the neuropil were decreased by about 70 percent. Two types of abnormal behavior -- clasping and footslips -- were ameliorated by approximately 50 percent and 70 percent, respectively. In addition, levels of the huntingtin messenger RNA, which encodes the protein that mediates Huntington's disease, were reduced by about 70 percent. Alnylam believes these findings support further studies of RNAi therapeutics for the treatment of Huntington's disease.

Neuropathic Pain

In a poster titled "RNAi of Neuropeptide Y for Neuropathic Pain," Alnylam collaborator Dr. Josephine Lai, Professor of Pharmacology at the University of Arizona, presented results showing that intrathecal injection of an siRNA targeting neuropeptide Y (NPY) prevented the development of neuropathic pain in a rat model. A more effective siRNA, designed by Alnylam, has been identified from in vitro studies and will be tested in vivo for enhanced activity. Further, in a talk titled "Treating Neuropathic Pain with RNA Interference," Dr. Lai presented results demonstrating that a single intraparenchymal injection of a very small dose of siRNA targeting the receptor of NPY in a lipid formulation was efficacious against neuropathic pain in a rat model. Alnylam believes these findings further support the use of RNAi as a highly potent therapeutic approach for the treatment of disorders of the nervous system.

About RNA Interference (RNAi)

RNA interference, or RNAi, is a naturally occurring mechanism within cells for selectively silencing and regulating specific genes. The discovery of RNAi has been widely acknowledged as a major breakthrough in biology, and the technology was recognized for its potential broad impact in medicine with the award of the 2006 Nobel Prize for Physiology or Medicine. Since many diseases are caused by the inappropriate activity of specific genes, the ability to silence genes selectively through RNAi could provide a new way to treat a wide range of human diseases. RNAi is induced by small, double-stranded RNA molecules. One method to activate RNAi is with chemically synthesized small interfering RNAs, or siRNAs, which are double-stranded RNAs that are targeted to a specific disease-associated gene. The siRNA molecules are used by the natural RNAi machinery in cells to cause targeted gene silencing.

About Alnylam

Alnylam is a biopharmaceutical company developing novel therapeutics based on RNA interference, or RNAi. The company is applying its therapeutic expertise in RNAi to address significant medical needs, many of which cannot effectively be addressed with small molecules or antibodies, the current major classes of drugs. Alnylam is building a pipeline of RNAi therapeutics; its lead program is in Phase I human clinical trials for the treatment of respiratory syncytial virus (RSV) infection, which is the leading cause of hospitalization in infants in the U.S. The company's leadership position in fundamental patents, technology, and know-how relating to RNAi has enabled it to form major alliances with leading companies including Merck, Medtronic, Novartis, and Biogen Idec. The company, founded in 2002, maintains global headquarters in Cambridge, Massachusetts, and has an additional operating unit in Kulmbach, Germany. For more information, visit www.alnylam.com.

Alnylam Forward-Looking Statements

Various statements in this release concerning our future expectations, plans and prospects, including without limitation, statements concerning the use of siRNAs to achieve therapeutic silencing of disease-causing genes, statements concerning effective and clinically relevant delivery of direct and systemic RNAi applications, statements concerning the timing of the filing of an IND application for ALN-PCS01, statements concerning the use of RNAi therapeutics for the treatment of Huntington's disease, and statements concerning the use of RNAi as a highly potent therapeutic approach for the treatment of disorders of the nervous system, constitute forward-looking statements for the purposes of the safe harbor provisions under The Private Securities Litigation Reform Act of 1995. Actual results may differ materially from those indicated by these forward-looking statements as a result of various important factors, including risks related to: our approach to discover and develop novel drugs, which is unproven and may never lead to marketable products; our ability to fund and the results of further pre-clinical and clinical trials; obtaining, maintaining and protecting intellectual property utilized by our products; our ability to enforce our patents against infringers and to defend our patent portfolio against challenges from third parties; our ability to obtain additional funding to support our business activities; our dependence on third parties for development, manufacture, marketing, sales, and distribution of products; the successful development of our product candidates, all of which are in early stages of development; obtaining regulatory approval for products; competition from others using technology similar to ours and others developing products for similar uses; our dependence on collaborators; and our short operating history; as well as those risks more fully discussed in the "Risk Factors" section of our most recent report on Form 10-Q on file with the Securities and Exchange Commission. In addition, any forward-looking statements represent our views only as of today and should not be relied upon as representing our views as of any subsequent date. Alnylam does not assume any obligation to update any forward-looking statements.