CAMBRIDGE, Mass. and DUBLIN, Ireland--(CCNMatthews - Mar 29, 2006) -Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc. (NYSE: ELN) announced today that they have enrolled and dosed the first patients in the TYSABRI® (natalizumab) monotherapy safety extension study program in multiple sclerosis (MS). Patients who previously participated in the Phase III MS trials and subsequent safety evaluation are eligible to be screened for entry in this open label multi-center study. Sites throughout Europe, the United States, Canada, Australia, New Zealand and Israel are expected to enroll patients. This safety extension study is being conducted under a U.S. Food and Drug Administration (FDA) Investigational New Drug (IND) application in the U.S. and similar investigational approvals internationally.