Tysabri works great for many of the people I know--which is about 10. The PML risk is there, and one should assume it fatal when it occcurs. The risk is higher for those who have received mutiple immune modulating therapies, especially when they have been given in close temporal proximity. If two immune modulating ("traditional") therapies fail to prevent flare ups then tysabri should be considered next along with other treatments--with the understanding of the risk of PML.