If JCV test gains validation, Not sure what you mean by validation. IF that meant that staying on T WOULD lead to PML then sure - people would drop out. I think seropositive patients already 2+ years on Tysabri will discontinue. It will be a hard decision. Go on with a small risk of death or go off and end up in a wheel chair in two years. I think we will see lots of people risk death. ij