Biogen Idec and Elan Provide Update on Utilization, Safety and Total Patient Exposure of TYSABRI(R) in Patients with Multiple Sclerosis

Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN)
today announced new data on the global utilization, safety and overall
patient exposure of TYSABRI(R) (natalizumab). As of late December
2007, more than 21,000 patients were on commercial and clinical
therapy worldwide. To date, the safety data continue to support a
favorable benefit-risk profile for TYSABRI. These data will be
presented today at 4:00 p.m. PST at the 26th Annual JPMorgan
Healthcare Conference in San Francisco.

According to data available to the companies as of late December
2007:

-- In the US, approximately 12,900 patients were on TYSABRI
therapy commercially and approximately 2,500 physicians have
prescribed the therapy;

-- Internationally, approximately 7,500 patients were on TYSABRI
therapy commercially;

-- In global clinical trials, approximately 700 patients were on
TYSABRI therapy; and

-- There have been no cases of progressive multifocal
leukoencephalopathy (PML) since re-launch in the US and launch
internationally in July 2006.

In addition, as of mid-December 2007:

-- Cumulatively, in the combined clinical trial and postmarketing
settings, up to 30,900 patients have been treated with
TYSABRI; and

-- Of those patients, up to 6,300 have received at least one year
of TYSABRI therapy.

TYSABRI is available in the United States through the TOUCH(TM)
Prescribing Program. All US prescribers, infusion sites and patients
receiving TYSABRI are required to enroll in TOUCH. Safety information
is also collected through ongoing clinical trials and registries,
including TYGRIS and the pregnancy registry, making this the largest
long-term patient follow-up effort undertaken for any MS therapy.

About TOUCH and TYGRIS

Before initiating treatment, all US patients, prescribers and
infusion sites must be enrolled in the TOUCH Prescribing Program
(TYSABRI Outreach: Unified Commitment to Health). TOUCH is designed to
determine the incidence of and risk factors for serious opportunistic
infections (OIs), including PML, and to monitor patients for signs and
symptoms of PML while promoting informed benefit-risk discussions
prior to initiating TYSABRI treatment. Physicians report on PML, other
serious OIs, deaths and discontinuation of therapy on an ongoing
basis.

TYGRIS (TYSABRI Global ObseRvation Program In Safety) is expected
to enroll 5,000 patients worldwide, including approximately 3,000
patients from TOUCH. Patients in TYGRIS are evaluated at baseline and
every six months thereafter for five years. Researchers will evaluate
data including medical/MS history; prior TYSABRI use; prior use of
immunomodulatory, antineoplastic, or immunosuppressive agents; and all
serious adverse events, including PML and other serious OIs, and
malignancies.

Adverse event reporting in the post-marketing setting is
voluntary. It is possible that not all reactions have been reported,
or that some reactions are not reported to Biogen Idec or Elan in a
timely manner.

About TYSABRI

TYSABRI is a treatment approved for relapsing forms of MS in the
United States and relapsing-remitting MS in the European Union.
According to data that have been published in the New England Journal
of Medicine, 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).

TYSABRI increases the risk of progressive multifocal
leukoencephalopathy (PML), an opportunistic viral infection of the
brain that usually leads to death or severe disability. Other serious
adverse events that have occurred in TYSABRI-treated patients included
hypersensitivity reactions (e.g., anaphylaxis) and infections. Serious
opportunistic and other atypical infections have been observed in
TYSABRI-treated patients, some of whom were receiving concurrent
immunosuppressants. Herpes infections were slightly more common in
patients treated with TYSABRI. 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. Common adverse events reported in TYSABRI-treated patients
include headache, fatigue, infusion reactions, urinary tract
infections, joint and limb pain, and rash.

In addition to the United States and European Union, TYSABRI is
also approved for MS in Switzerland, Canada, Australia, New Zealand
and Israel. TYSABRI was discovered by Elan and is co-developed with
Biogen Idec.

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 therapeutic areas
with high unmet medical needs. Founded in 1978, Biogen Idec is a
global leader in the discovery, development, manufacturing, and
commercialization of innovative therapies. Patients in more than 90
countries benefit from Biogen Idec´s significant products that address
diseases such as lymphoma, multiple sclerosis, and rheumatoid
arthritis. 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 the 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, that the
companies may encounter other unexpected hurdles, or that new
therapies for MS with better efficacy or safety profiles or more
convenient methods of administration are introduced into the market.
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.