Biogen Idec’s Ongoing Dedication to People Living with MS Showcased in Data Presented at the 63rd Annual Meeting of the American Academy of Neurology

-- Company Continues to Redefine the Future of Multiple Sclerosis Treatment --

Biogen Idec (NASDAQ: BIIB) today announced that data highlighting the company’s focus and drive towards addressing the needs of people living with multiple sclerosis (MS) will be presented at the American Academy of Neurology’s (AAN) 63^rd Annual Meeting in Honolulu, April 9-16, 2011. Fifty-five company-sponsored platform and poster presentations featuring data focusing on the company’s innovation and research in the treatment and management of the disease will be presented during the Congress. Highlights will include data on Biogen Idec’s currently marketed products, TYSABRI^® (natalizumab) and AVONEX^® (interferon beta-1a), as well as four late-stage programs: FAMPYRA^® (prolonged-release fampridine tablets), the oral disease-modifying therapy BG-12 (dimethyl fumarate), PEGylated interferon beta-1a and daclizumab.

“The data being presented at AAN illustrate the breadth of Biogen Idec’s ongoing commitment to the MS community,” said Douglas E. Williams, Ph.D., executive vice president, research and development at Biogen Idec. “MS affects each person differently and, until we find a cure, Biogen Idec is dedicated to driving ground-breaking research and pursuing highly differentiated therapies. Our leading marketed products, TYSABRI and AVONEX continue to remain gold-standards of care in the treatment and management of MS. Our robust pipeline has the potential to offer effective and convenient therapies targeting the different pathways of the disease, addressing the unique needs of each individual MS patient.”

The following represents select data highlights during the meeting from the Company’s portfolio of marketed products.

TYSABRI

Key TYSABRI data being presented at the Congress includes:

• Assessment of Baseline Treatment History and Postbaseline Relapses and Serious Adverse Events in MS Patients Treated with Natalizumab – S51.005
• Risk Stratification for Progressive Multifocal Leukoencephalopathy (PML) in MS Patients: Role of Prior Immunosuppressant Use, Natalizumab-Treatment Duration, and Anti-JCV Antibody Status – P03.248
• Assessment of the Association Between Baseline Characteristics and Postbaseline Relapses, Disability Progression and Improvement over Time in the Natalizumab (TYSABRI) Observational Program (TOP) in Patients with Multiple Sclerosis – P01.199
• Patient-Reported Disability Level and Functional Status: Impact after One Year of Natalizumab Treatment – P07.155
• Effects of Natalizumab on Cognition in Multiple Sclerosis: Findings from the ENER-G Study – P06.077
• Effects of Natalizumab on Bladder Function: Interim Results of the TRUST Study – PD06.009

AVONEX

Key AVONEX data being presented at the Congress includes:

• Cost-Effectiveness of Injectable Disease-Modifying Therapies in the Treatment of Multiple Sclerosis – P06.040
• Association between Changes in Gray Matter MRI Measures and Disability Progression over 5 Years in Patients Treated with IM IFNB-1a – P05.058

Dr. Williams continued, “Over the last several years, we have consistently expanded and redefined patients’ expectations for successful management of their multiple sclerosis. We continue to make significant investments in the discovery and development of new treatments, while remaining committed to increasing the safety and efficacy of available therapies. As a company, we recognize the unmet need that remains within the MS community and believe that no one is doing more to address it than Biogen Idec.”

The following represents select data highlights during the meeting from the Company’s late-stage MS portfolio.

FAMPYRA (prolonged-release fampridine tablets)

Key FAMPYRA data being presented at the Congress includes:

• Dalfampridine Extended Release Tablets Improved Walking Speed in Patients with Severe Walking Impairment – P07.172
• “Responder or Non-Responder, That is the Question” Was the Responder Definition Used in the Dalfampridine Extended Release Studies Clinically Meaningful? – P01.202

BG-12 (dimethyl fumarate)

Key BG-12 data being presented at the Congress includes:

• Neuroprotective Effects of BG-12 and Other Fumarates on Primary Cultures of Neurons and Astrocytes after Oxidative Challenge – P05.037
• BG-12: A Novel Therapeutic to Promote Oligodendrocyte Survival? – P02.183

PEGylated Interferon beta-1a

Key PEGylated Interferon beta-1a data being presented at the Congress includes:

• Evaluation of IFN Beta-1a Induction of miRNA/mRNA in Healthy Control Subjects Using a Novel High-Throughput Whole-Blood Expression-Profiling Method – P02.164
• Impact of Renal Function on the Clearance of PEGylated Interferon Beta-1a – P03.238

Daclizumab

Key daclizumab data being presented at the Congress includes:

• Subcutaneous Daclizumab, Injection-Site Reactions, and Anti-drug Antibodies: Results from the Phase 2 CHOICE Trial – P04.200

About Biogen Idec

Biogen Idec uses cutting edge science to discover, develop, manufacture and market therapeutic products for the treatment of serious diseases with a focus on neurological disorders. Founded in 1978, Biogen Idec is the world's oldest independent biotechnology company. Patients worldwide benefit from its leading multiple sclerosis therapies and the company generates more than $4 billion in annual revenues. For product labeling, press releases and additional information about the company, please visit www.biogenidec.com .

About TYSABRI

TYSABRI is approved in more than 45 countries. In the U.S., it is approved for relapsing forms of multiple sclerosis (MS) and in the European Union for relapsing-remitting MS (RRMS).

TYSABRI has advanced the treatment of MS patients with its established efficacy. It has been proven to reduce flare-ups and slow physical disability progression. Data from the Phase 3 AFFIRM trial, which was published in the New England Journal of Medicine , showed that after two years, TYSABRI treatment led to a 68 percent relative reduction (p<0.001) in the annualized relapse rate when compared with placebo and reduced the relative risk of disability progression by 42-54 percent (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.

The risk of PML increases with longer treatment duration and in patients treated with an immunosuppressant prior to receiving TYSABRI. Other serious adverse events that have occurred in TYSABRI-treated patients include hypersensitivity reactions (e.g., anaphylaxis) and infections, including opportunistic and other atypical infections.

Clinically significant liver injury has also been reported in patients treated with TYSABRI in the post-marketing setting. Common adverse events reported in TYSABRI-treated MS patients include headache, fatigue, infusion reactions, urinary tract infections, joint and limb pain, and rash. TYSABRI is co-marketed by Biogen Idec Inc. and Elan Corporation, plc. For more information about TYSABRI, please visit www.tysabri.com , www.biogenidec.com or www.elan.com , or call 1-800-456-2255.

About AVONEX

AVONEX is one of the most prescribed treatments for relapsing forms of MS worldwide. It is used worldwide as a treatment for relapsing forms of MS to slow the progression of physical disability and reduce relapses. AVONEX is also approved for patients who have their first clinical MS attack and have a brain MRI scan consistent with MS.

The most common side effects associated with AVONEX MS treatment are flu-like symptoms, including myalgia, fever, fatigue, headache, chills, nausea, vomiting, pain and asthenia.

AVONEX should be used with caution in patients with depression or other mood disorders and in patients with seizure disorders. AVONEX should not be used by pregnant women. Patients with cardiac disease should be closely monitored. Patients should also be monitored for signs of hepatic injury. Routine periodic blood chemistry and hematology tests are recommended during treatment with AVONEX. Rare cases of anaphylaxis have been reported. Please see complete prescribing information available at www.AVONEX.com .

About FAMPYRA

FAMPYRA 10 mg tablets is a prolonged-release (sustained release) tablet formulation of the drug fampridine (4-aminopyridine or dalfampridine). FAMPYRA has been developed to improve walking in adult patients with MS. In MS, damaged myelin exposes channels in the membrane of axons allowing potassium ions to leak, weakening the electrical current sent through nerves. Studies have shown that fampridine can increase conduction along damaged nerves, which may result in improved walking ability. This prolonged-release formulation was developed and is being commercialized in the U.S. by Acorda Therapeutics, Inc. It has been approved in the U.S. under the trade name AMPYRA^® (dalfampridine) Extended Release Tablets, 10 mg. Biogen Idec plans to commercialize and further develop the product outside of the U.S. under a licensing agreement with Acorda.

About BG-12

BG-12 (dimethyl fumarate) is an investigational oral therapy in clinical development for the treatment of RRMS, the most common form of MS. BG-12 is currently being evaluated as a monotherapy in two Phase 3 clinical trials, DEFINE and CONFIRM, and in combination with commonly used first-line treatments in the Phase 2 EXPLORE trial.

Top-line results from the DEFINE study showed that 240 mg of BG-12, administered either twice or three times a day, demonstrated a highly statistically significant reduction (p<0.0001 ) in the proportion of patients with RRMS who relapsed at two years compared with placebo. Both doses of BG-12 also demonstrated a statistically significant reduction in annualized relapse rate, in the number of new or newly enlarging T2 hyperintense lesions, in new gadolinium-enhancing lesions, and in the rate of disability progression as measured by the Expanded Disability Severity Scale at two years. Initial data from the trial also showed that BG-12 demonstrated a favorable safety and tolerability profile.

BG-12 received Fast Track designation in MS from the U.S. Food and Drug Administration (FDA) in 2008. Biogen Idec retains full worldwide commercial rights to BG-12.

About PEGylated Interferon Beta-1a

PEGylated interferon beta-1a is under investigation for the treatment of relapsing MS and is currently enrolling a Phase 3 clinical trial. PEGylated interferon beta-1a, administered via subcutaneous injection, is being evaluated for its ability to last longer in a patient's system, potentially leading to an MS treatment that would require fewer injections. Patients interested in learning more about the ADVANCE trial may speak with their physician or email ADVANCEstudy@biogenidec.com .

About Daclizumab

Daclizumab is a humanized monoclonal antibody that binds to the CD25 alpha subunit of the high affinity IL-2 receptor. CD25 is expressed at low levels on resting T-cells (immune cells) and at high levels on T-cells that can become activated in response to autoimmune conditions such as MS. Daclizumab is believed to work by selectively binding to and inhibiting this receptor on activated T-cells without causing T-cell depletion. Daclizumab is an investigational agent in clinical development for the treatment of MS in collaboration between Abbott and Biogen Idec. Daclizumab is currently being studied in two registrational clinical trials in patients with MS. Patients interested in learning more about the DECIDE trial may speak with their physician or email DECIDEstudy@biogenidec.com .

Safe Harbor

This press release contains forward-looking statements, including statements about the development initiatives and growth strategies for our marketed products and the anticipated development of programs in our clinical pipeline. These forward-looking statements may be accompanied by such words as "anticipate," "believe," "estimate," "expect," "forecast," "intend," "may," "plan," "will" and other words and terms of similar meaning. You should not place undue reliance on these statements.

These statements involve risks and uncertainties that could cause actual results to differ materially from those reflected in such statements, including our dependence on our three principal products, AVONEX, RITUXAN and TYSABRI, the importance of TYSABRI's sales growth, product competition, uncertainty of success in commercializing other products, the occurrence of adverse safety events with our products, changes in the availability of reimbursement for our products, adverse market and economic conditions, our dependence on collaborations and other third parties over which we may not always have full control, failure to execute our growth initiatives, failure to comply with government regulation and possible adverse impact of changes in such regulation, charges and other costs relating to our properties, problems with our manufacturing processes and our reliance on third parties, fluctuations in our effective tax rate, our ability to attract and retain qualified personnel, the risks of doing business internationally, our ability to protect our intellectual property rights and the cost of doing so, product liability claims, fluctuations in our operating results, the market, interest and credit risks associated with our portfolio of marketable securities, our level of indebtedness, environmental risks, aspects of our corporate governance and collaborations, representation of activist shareholders on our board of directors, and the other risks and uncertainties that are described in the Risk Factors section of our most recent annual or quarterly report and in other reports we have filed with the SEC.

These statements are based on our current beliefs and expectations and speak only as of the date of this press release. We do not undertake any obligation to publicly update any forward-looking statements.

Contact:

Biogen Idec
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Shannon Altimari, +41 (0) 41 392 16 77
Senior Manager, Public Affairs
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