Data Presented at the American Academy of Neurology's Annual Meeting Provide Update on Utilization and Safety of TYSABRI(R) in Patients with Multiple Sclerosis

Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN)
announced today that new data from the TOUCH Prescribing Program(TM)
and TYGRIS safety study confirm the safety profile from previous
clinical studies of TYSABRI(R) (natalizumab). Also presented at the
59th annual meeting of the American Academy of Neurology in Boston, MA
were extension study data that showed that TYSABRI has a sustained
treatment effect on clinical relapses and the risk of disability
progression in multiple sclerosis (MS) patients treated for up to
three years. The companies recently reported that as of mid-April 2007
approximately 12,500 patients have been prescribed TYSABRI worldwide.
The companies estimate that in both commercial use and clinical
trials, there are currently over 10,000 patients on TYSABRI therapy
worldwide.

"The findings from the safety update combined with the data
showing the sustained effect of TYSABRI in patients treated for up to
three years, contribute to our evolving understanding of the
utilization of this therapy as an important treatment option for
people living with the debilitating effects of MS," said Paul
O'Connor, MD, St. Michael's Hospital, Toronto, Ontario, Canada, lead
investigator of the TYSABRI extension study.

TYSABRI Update

TYSABRI is available in the US through the TOUCH Prescribing
Program. All 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
STRATA, TYGRIS and the pregnancy registry. According to data available
to the companies as of April 23, 2007, there have been no new reports
of confirmed cases of progressive multifocal leukoencephalopathy (PML)
or other serious opportunistic infections (OIs). The data confirm the
safety profile from previous clinical studies of TYSABRI and will
continue to expand the knowledge of the long-term safety and
tolerability of TYSABRI.

The combination of TOUCH, TYGRIS and the pregnancy registry will
be the largest long-term follow-up undertaken for an MS therapy, and
the companies plan to continue to provide similar updates at upcoming
medical meetings.

The companies recently announced that as of mid-April,
approximately 12,500 patients have been prescribed TYSABRI worldwide.
In both commercial use and clinical trials, there are currently over
10,000 patients on TYSABRI therapy worldwide.

-- In the US, approximately 6,600 patients are on TYSABRI therapy
commercially. Approximately 10,000 patients have enrolled in
the TOUCH program and 1,500 physicians have enrolled patients.

-- In the EU, approximately 2,500 patients internationally have
received TYSABRI infusions commercially, mostly in Germany and
the Nordic countries.

-- In clinical trial settings, over 1,000 patients are on TYSABRI
therapy.

TYSABRI Efficacy Sustained through Three Years

Patients who participated in the Phase III TYSABRI program were
eligible to enroll in an open-label extension study that evaluated the
therapy's long-term effects. Included in this were patients from
AFFIRM, a randomized, double-blind, placebo-controlled, two-year
monotherapy study of TYSABRI that enrolled 942 patients (627 patients
on TYSABRI, 315 on placebo). In AFFIRM, TYSABRI reduced the annualized
relapse rate in patients with MS by 67% (p<0.001) and the risk of
12-week sustained disability progression by 42% (p<0.001) compared
with placebo.

In the intent to treat analysis, the annualized relapse rate for
patients treated with TYSABRI over the three-year period was 0.23,
translating into an average of one relapse every 4.3 years. The
relapse rate also continued to remain low over the three-year
treatment period with TYSABRI: 0.27 during the first year; 0.20 during
the second year; and 0.15 during the third year (based on 531 patients
who entered the extension study, which includes approximately 250
patients with nearly three years of continuous therapy).

In addition, TYSABRI also decreased the cumulative probability of
disability progression sustained for six months compared to placebo.
The estimated proportion of patients who had 24-week sustained
disability progression at two years was 11% in patients treated with
TYSABRI compared to 23% in patients treated with placebo, a 54%
relative reduction.

This effect was maintained in patients treated with TYSABRI for up
to three years with 13% showing 24-week sustained disability
progression.

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 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, serious OIs, deaths and
discontinuation of therapy on an ongoing basis.

TYGRIS (TYSABRI Global ObseRvation Program In Safety) is expected
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.

The information provided here is derived from voluntary adverse
event reporting. 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

In the US, TYSABRI is approved as a monotherapy treatment for
relapsing forms of MS. TYSABRI increases the risk of 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 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 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.