I misspoke, and Im sorry. the research I saw said it was 10.6 per 1000 cases, but I was unable to find the paper I saw.

I did however see some documents that say if you have had prior suppression therapy your risk is 11 in 1000 as you reach your 24th treatment. if you have never had therapy to suppress before your risk is 4 in 1000 as you reach your 24th therapy. They are seeing the longer you are on it, the higher the risk, but most occcur between 24 and 36 months of treatment. you MUST be positive for the JC virus to develop PML.

Click on link then look for RISK factors with PML on the right side tab. Scroll down to blue chart.
http://www.tysabri.com/pml-risk.xml#test-results