"Earlier this year, Biogen Idec (nasdaq: BIIB - news - people ) and Elan (nyse: ELN - news - people ) pulled their multiple sclerosis drug, Tysabri, after the drug was linked to a disorder called progressive multifocal leukoencephalopathy (PML). Three of the 3,000 patients who took Tysabri in clinical trials developed PML, which is caused by the JC virus, which is often dormant in many patients. Today, the New England Journal of Medicine published case reports on all three of those patients. Alongside the reports, which provide a great deal of detail on how PML developed, the journal also published an editorial by Joseph Berger and Igor Koralnik, two of the world's top experts in PML who have worked as consultants for Biogen Idec and Elan. The two write that the linkage between the drug and PML, though unexpected seems clear. But they also provide a bit of light to investors, writing that it may be possible to detect the proliferation of JC virus before much of the damage is done and then stop Tysabri treatment. It remains an open question whether the Food and Drug Administration will require a new clinical trial to prove that is the case. Biogen Idec says it hopes to finish its safety review by the end of the summer. Shares of Biogen Idec and Elan were both up 4% ahead of the announcement" FORBES MAGAZINE