Tysabri - PML: Another Partner for Biogen : "Biogen isn't licensing any drug in clinical trials, but rather paying for Alnylam to do research in hopes of developing a drug candidate to treat a very rare disease called progressive multifocal leukoencephalopathy (PML)....If the acronym PML rings a bell to Biogen investors, it's because PML is the ultra-rare condition, usually occurring in people with immune deficiencies, that caused Tysabri, the company's innovative multiple sclerosis drug, to be removed from the market for more than a year after PML occurred in three patients who...."