Although the cause of MS is unknown, it is widely considered to be an autoimmune disease in which the person's immune system attacks the brain and/or spinal cord, the FDA says. Tysabri appears to work by binding to these immune system cells, thus preventing them from traveling to the brain where they can cause damage. In the first clinical trial of the product's safety and efficacy, the drug reduced the frequency of relapses by 66% relative to placebo, the FDA announcement said. In another clinical trial, patients who had been treated with Biogen's MS drug Avonex but who had experienced one or more relapses while on Avonex were given either Tysabri or placebo. Avonex was continued throughout the study for both groups. In this trial, Tysabri reduced the frequency of relapses by 54% relative to placebo. For Biogen Idec, the big question is whether -- or by how much -- Tysabri will cannibalize sales of Avonex. Analysts have said they expect Tysabri to perform well at the expense of other MS drugs, including Rebif from Serono ( SRA); Betaseron from Chiron ( CHIR); and Copaxone from Teva Pharmaceutical Industries ( TEVA).