I saw this article today and thought I would share it. The authors suggest there may be a connection between Tysabri and the development of melanoma. Basically, two women who had pre-existing moles received Tysabri for MS and rapidly the moles transformed into an aggressive form of metastatic melanoma. It only presents two cases (there are a total of 3 reported), obviously many more people have taken it. Two important points are that (1) the article suggests that animal studies showed something similar, so it could be a real cause/effect relationship--not a coincidence--though it is too early to say; and (2) if you don't have moles or a family history of melanoma, you are probably not at risk.
If you are considering Tysabri, this is something you may want to discuss with your doctor.