Cambridge, MA; San Diego, CA; Dublin, Ireland November 23, 2004 Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN) announced today that the U.S. Food and Drug Administration (FDA) has approved TYSABRI (natalizumab), formerly referred to as ANTEGREN, as treatment for relapsing forms of multiple sclerosis (MS) to reduce the frequency of clinical relapses. FDA granted Accelerated Approval for TYSABRI following Priority Review based on one-year data from two Phase III studies, the AFFIRM monotherapy trial and the SENTINEL add-on trial with AVONEX(Interferon beta-1a).

TYSABRI, the first humanized monoclonal antibody approved for the treatment of MS, inhibits adhesion molecules on the surface of immune cells. Research suggests TYSABRI works by preventing immune cells from migrating from the bloodstream into the brain where they can cause inflammation and potentially damage nerve fibers and their insulation.

"TYSABRI is a powerful and innovative therapy that offers new hope for hundreds of thousands of people living with MS," said James C. Mullen, chief executive officer, Biogen Idec. "We believe TYSABRI will revolutionize the treatment of MS and become the leading choice for patients and physicians."

"TYSABRI is a significant breakthrough for patients with MS," said Kelly Martin, president and chief executive officer, Elan. "The approval of TYSABRI, with its unique mechanism of action and new level of efficacy, has the potential to make a genuine difference in the lives of patients and families who struggle with the debilitating effects of this disease."

Results of the AFFIRM Monotherapy Trial

AFFIRM is a two-year, randomized, multi-center, placebo-controlled, double-blind study of 942 patients conducted in 99 sites worldwide, in which patients were
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Contact: Amy Losak
646-935-9317
Ketchum
23-Nov-2004