The FDA recently released some new info on PML and Tysabri. The estimated incidence of PML per 1,000 patients is 0.3 for the first 24 infusions, 1.5 for infusion 25-36 and 0.9 for infusions 37-48.
"The risk of developing PML also increases if you have taken a medication that suppresses the immune system (called an immunosuppressant) in the past. Tell your healthcare professional if you have previously taken an immunosuppressant medication. Examples of immunosuppressants are mitoxantrone, azathioprine, methotrexate, cyclophosphamide, and mycophenolate."
http://www.fda.gov/Drugs/DrugSafety/ucm252045.htm
"The risk of developing PML also increases if you have taken a medication that suppresses the immune system (called an immunosuppressant) in the past. Tell your healthcare professional if you have previously taken an immunosuppressant medication. Examples of immunosuppressants are mitoxantrone, azathioprine, methotrexate, cyclophosphamide, and mycophenolate."
http://www.fda.gov/Drugs/DrugSafety/ucm252045.htm