Quoted from http://online.wsj.com/article/SB124631389418370485.html
Biogen Reports a 13th Sick Patient on MS Drug
By KEITH J. WINSTEIN
Biogen Idec Inc.'s disclosure of a debilitating brain infection in a patient taking its Tysabri multiple-sclerosis medicine sparked a selloff in shares of the biotechnology company Monday.
The disclosure, made Friday evening, marks the 13th case of the infection, called progressive multifocal leukoencephalopathy or PML, in a Tysabri patient. Three of the patients have died....
Tysabri was pulled from the U.S. market in 2005 after three initial cases. After holding hearings to weigh Tysabri's risks and benefits, the Food and Drug Administration allowed it to be reintroduced in 2006, under monitoring rules and a prohibition on using it in combination with other MS drugs. The system was aimed at reducing PML incidence.