From: Gordon Francis, MD, Vice President, Neurology Therapeutic Market, Elan Pharmaceuticals:

"Dear Healthcare Professional,

In an effort to keep you informed of new developments, Biogen Idec and Elan
Pharmaceuticals are providing an update related to the ongoing safety evaluation of
TYSABRI® (natalizumab). On August 9, 2005, we announced that findings from our
ongoing safety evaluation of TYSABRI revealed no new confirmed cases of progressive
multifocal leukoencephalopathy (PML) in multiple sclerosis (MS). The Crohn’s disease and
rheumatoid arthritis safety evaluation is on track to be completed by the end of the summer.

More than 2,000 MS clinical trial patients were eligible for the TYSABRI safety evaluation.
91% of these patients chose to participate in the evaluation. Of these participating patients,
99% had a neurological exam, and 98% had an MRI. The safety evaluation also included the
review of any reports of potential PML in patients receiving TYSABRI in the commercial
setting.

On February 28, 2005, we announced the voluntary suspension of TYSABRI from the US
market and all ongoing clinical trials based on reports of 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. We remain committed to
completing this safety evaluation and look forward to working with regulatory agencies to
determine the appropriate path forward for TYSABRI. Based on expected completion of the
safety assessment by the end of the summer, we anticipate making regulatory submissions to
the U.S. Food and Drug Administration (FDA) by early fall.

Thank you for your patience and continued support over the past few months. We will
continue to inform you of new developments and changes. For additional questions, please
contact Medical Information at"