This recent NMSS link does not detail they got to a number of 24, EXCEPT to say that there are 24 confirmed cases "among people who have used Tysabri® after it became available for prescription in July 2006:

http://www.nationalmssociety.org/new....aspx?nid=2308

However, before it became availabe for prescription, (i.e. the trial patients), there were also the first 3 cases. 24 + 3 = 27

For the current ongoing counts that are kept though, they always knock off the original trial patient PML cases, although I am not sure why. At one time they thought those 3 were "exceptional" cases, perhaps because they were on combined therapy vs. monotherapy .... however we know now that PML is going to occur anyway so it's probably "6 of one, 1/2 dozen of another".

Cherie