Deerfield, IL and Cambridge, MA, April 3, 2006 – Astellas Pharma US, Inc. and Biogen Idec Inc. (NASDAQ: BIIB) today announced that an agreement for Astellas to purchase the worldwide rights to AMEVIVE® (alefacept) was executed as of March 31, 2006. AMEVIVE is a biologic anti-inflammatory compound used in the treatment of moderate-to-severe plaque psoriasis.

Under the terms of the agreement, Astellas would acquire the worldwide rights to AMEVIVE in all indications from Biogen Idec. Biogen Idec would continue to manufacture AMEVIVE and supply product to Astellas. The parties anticipate closing the transaction as early as mid-April.

“The acquisition of a well-established product such as AMEVIVE strengthens our North American Dermatology franchise and provides Astellas an opportunity to continue offering physicians this effective option for the treatment of moderate-to-severe plaque psoriasis,” stated William Fitzsimmons, Pharm.D., Senior Vice President, Business Development at Astellas Pharma US, Inc.

About AMEVIVE

AMEVIVE is the first biologic approved by the FDA for the treatment of moderate to severe chronic plaque psoriasis in adults who are candidates for systemic therapy or phototherapy. AMEVIVE is a prescription medication that is injected by your doctor (15 mg IM, into the muscle) once a week, for a total of 12 doses. AMEVIVE was approved by the US FDA in January 2003. The most serious adverse reactions found in clinical trials were: lymphopenia, malignancies, serious infections requiring hospitalization and hypersensitivity. Amevive is contraindicated in patients infected with HIV and in patients who are allergic to any of the components of Amevive For more information about AMEVIVE, including prescribing information call 1-866-AMEVIVE or visit www.amevive.com.

About Chronic Plaque Psoriasis

Psoriasis is a chronic but treatable autoimmune skin disease. Roughly two percent of the world’s population has psoriasis. In the US, psoriasis affects 4.5-6 million people, and about 1.2 million people have a moderate to severe form of chronic plaque psoriasis. While psoriasis is not typically life threatening, it can greatly affect appearance, self-esteem, and quality of life. Chronic plaque psoriasis can develop anywhere on the skin, though it usually appears on the scalp, knees, elbows, and trunk.

About Astellas Pharma US, Inc.

Astellas Pharma US, Inc. is a subsidiary of Astellas Pharma Inc., located in Tokyo, a pharmaceutical company dedicated to improving the health of people around the world through the provision of innovative and reliable pharmaceutical products. In April 2005, the company was formed through the merger of Fujisawa Pharmaceutical Co., Ltd. and Yamanouchi Pharmaceutical Co., Ltd. The organization is committed to becoming a global mega pharmaceutical company by combining outstanding R&D and marketing capabilities and continuing to grow in the world pharmaceutical market. For more information about Astellas, please visit www.astellas.com/us.

About Biogen Idec Inc.

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 press releases and additional information about the company, please visit www.biogenidec.com.