New Data on TYSABRI(R) Presented at ECTRIMS Congress Demonstrate Significant Improvement in Cognitive Function in Patients with Multiple Sclerosis

Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN)
announced today that data from the Phase III AFFIRM monotherapy study
demonstrated that treatment with TYSABRI(R) (natalizumab)
significantly reduced the proportion of multiple sclerosis (MS)
patients with worsening cognitive function as measured by the 3-second
Paced Auditory Serial Addition Test (PASAT 3). These data, presented
at the 22nd Congress of the European Committee for Treatment and
Research in Multiple Sclerosis (ECTRIMS) in Madrid, Spain, contribute
to existing data which demonstrate the overall therapeutic benefits of
TYSABRI, including its significant impact on relapse reduction,
disability progression and MRI measures. TYSABRI has demonstrated a
68% relative reduction in the annualized relapse rate compared to
placebo and a 42% reduction in the relative risk of disability
progression, as published in the New England Journal of Medicine.

Cognitive deficits are under-recognized and often misdiagnosed as
depression, stress or other personality disorders. Studies have shown
that approximately 43% to 65% of MS patients show measurable cognitive
impairment in formal testing.(1) Cognitive dysfunction can occur early
in MS and in patients with relatively mild physical disability. These
deficits have a substantial effect on the daily functioning of
patients. Areas impacted by cognitive dysfunction include memory,
ability to process information and learning. (1),(2)

The AFFIRM study was a two-year, randomized, multi-center,
placebo-controlled, double-blind study of 942 patients conducted in 99
sites worldwide, evaluating the effect of TYSABRI on the progression
of disability and the rate of clinical relapses. Evaluating the effect
of TYSABRI on cognitive function was a pre-specified endpoint of the
AFFIRM study. Cognitive function was assessed using the 3-second Paced
Auditory Serial Addition Test (PASAT 3), a test of auditory
information processing. The study showed that treatment with TYSABRI
reduced the risk of sustained cognitive worsening by 43% (p=0.013)
when compared to placebo.

These cognitive function data complement the previously presented
results of the AFFIRM study, which demonstrated a significant effect
of TYSABRI on two-widely accepted health-related quality of life
measures, the Short Form-36 Health Survey and the Visual Analogue
Scale.

"Neuropsychological dysfunction significantly diminishes quality
of life in many patients with multiple sclerosis, impacting everything
from employment to social interaction. It is responsible for much
hardship experienced by MS patients. The important positive effects of
TYSABRI on cognitive functioning and quality of life add to the
important benefits already reported on progression of disability and
relapses. This provides strong evidence that observed neurologic
benefits translate into important improvements as perceived by the
patients," said Richard Rudick, MD, Director of the Mellen Center for
Multiple Sclerosis Treatment and Research at the Cleveland Clinic.

About TYSABRI

In the US, TYSABRI is approved as a monotherapy treatment for
relapsing forms of MS. TYSABRI increases the risk of progressive
multifocal leukoencephalopathy (PML), an opportunistic viral infection
of the brain that usually leads to death or severe disability.
Patients should be monitored at regular intervals for any new or
worsening signs or symptoms suggestive of PML. Because of the
increased risk of PML, TYSABRI is generally recommended for patients
who have had an inadequate response to, or are unable to tolerate,
alternate MS therapies. It is available in the US only through a
restricted distribution program called the TOUCH Prescribing Program.
According to product labeling, after two years, TYSABRI treatment led
to a 67% relative reduction (p<0.001) in the annualized relapse rate
compared to placebo and reduced the relative risk of disability
progression by 42% (p<0.001). TYSABRI treatment also resulted in
sustained and statistically significant reductions in brain lesion
activity as measured by MRI. Changes in MRI findings often do not
correlate with changes in the clinical status of patients (e.g.,
disability progression). The prognostic significance of the MRI
findings in these studies has not been evaluated.

In the European Union, TYSABRI is indicated as a single
disease-modifying therapy in highly active relapsing-remitting MS
patients. Because of the increased risk of PML, it is for patients
with high disease activity despite treatment with a beta-interferon or
in patients with rapidly evolving severe relapsing-remitting MS.
According to product labeling in the EU, after two years, TYSABRI
treatment led to a 68% relative reduction (p<0.001) in the annualized
relapse rate compared to placebo and reduced the relative risk of
disability progression by 42-54% (p<0.001).

Serious adverse events that occurred in TYSABRI-treated patients
included hypersensitivity reactions (e.g., anaphylaxis), infections,
depression and gallstones. In MS trials, the incidence and rate of
other serious and common adverse events, including the overall
incidence and rate of infections, were balanced between treatment
groups. Herpes infections were slightly more common in patients
treated with TYSABRI. Serious opportunistic and other atypical
infections have been observed in TYSABRI-treated patients, some of
whom were receiving concurrent immunosuppressants. Common adverse
events reported in TYSABRI-treated patients include headache, fatigue,
infusion reactions, urinary tract infections, joint and limb pain,
lower respiratory infections, rash, gastroenteritis, abdominal
discomfort, vaginitis, and diarrhea.

For more information about TYSABRI please visit www.tysabri.com,
www.biogenidec.com or www.elan.com, or call 1-800-456-2255.

About Biogen Idec

Biogen Idec creates new standards of care in oncology, neurology
and immunology. As a global leader in the development, manufacturing,
and commercialization of novel therapies, Biogen Idec transforms
scientific discoveries into advances in human healthcare. For product
labeling, press releases and additional information about the company,
please visit www.biogenidec.com.

About Elan

Elan Corporation, plc is a neuroscience-based biotechnology
company committed to making a difference in the lives of patients and
their families by dedicating itself to bringing innovations in science
to fill significant unmet medical needs that continue to exist around
the world. Elan shares trade on the New York, London and Dublin Stock
Exchanges. For additional information about the company, please visit
www.elan.com.

Safe Harbor/Forward Looking Statements

This press release contains forward-looking statements regarding
TYSABRI. These statements are based on the companies' current beliefs
and expectations. The commercial potential of TYSABRI is subject to a
number of risks and uncertainties. Factors which could cause actual
results to differ materially from the companies' current expectations
include the risk that we may be unable to adequately address concerns
or questions raised by FDA or other regulatory authorities, that
concerns may arise from additional data, that the incidence and/or
risk of PML or other opportunistic infections in patients treated with
TYSABRI may be higher than observed in clinical trials, or that the
companies may encounter other unexpected hurdles. Drug development and
commercialization involves a high degree of risk. For more detailed
information on the risks and uncertainties associated with the
companies' drug development and other activities, see the periodic and
current reports that Biogen Idec and Elan have filed with the
Securities and Exchange Commission. The companies assume no obligation
to update any forward-looking statements, whether as a result of new
information, future events or otherwise.

(1) Rao SM, et al. Neurology. 1991;41:685-691

(2) Amato MP, et al. Arch Neurol. 1995;52:168-172