Biogen Idec - link for news releaseCambridge, MA and Dublin, Ireland - July 18, 2005 - Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN) announced today that SENTINEL, the Phase III TYSABRI (natalizumab) add-on trial with AVONEX(Interferon beta-1a), achieved the two-year primary endpoint of slowing the progression of disability in patients with relapsing forms of multiple sclerosis (MS). The addition of TYSABRI to AVONEX resulted in a 24 percent reduction in the risk of disability progression compared to the effect provided by AVONEX alone. Data from SENTINEL also demonstrated that the addition of TYSABRI to AVONEX led to a 56 percent relative reduction in the rate of clinical relapses compared to that provided by AVONEX alone. The reduction in relapse rate was statistically significant and sustained over the entire two-year study period.On February 28, 2005, Biogen Idec and Elan announced that they voluntarily suspended TYSABRI from the U.S. market and all ongoing clinical trials based on reports of progressive multifocal leukoencephalopathy (PML), a rare and potentially fatal, demyelinating disease of the central nervous system. The companies have previously reported three confirmed cases of PML, two of which were fatal. "These data demonstrate the effect of TYSABRI on disability progression and clinical relapses. We continue to believe in the therapeutic benefit of TYSABRI in MS, a disease with significant unmet medical need," said Burt Adelman, MD, executive vice president, Development, Biogen Idec. "Our extensive safety evaluation, in collaboration with leading experts and regulatory agencies, is on track and we hope to have findings by the end of the summer.TYSABRI continues to show benefit in the treatment of immune-mediated diseases," said Lars Ekman, MD, PhD, executive vice president and president, Research and Development, Elan.

"Patient safety is our top priority. We remain strongly committed to defining TYSABRI's benefit-risk profile and determining the appropriate path forward."