the original 3 cases of PML were indeed initially considered to be due to the
use of
Tysabri in conjunction with other treatments for MS, particularly interferon. After being withdrawn from the market, Tysabri was brought back under the TOUCH program (which is strictly controlled by the FDA), and the thought was that if Tysabri was used as a monotherapy, the risk for PML would not be as significant.
Unfortunately, this proved not to be the case.