Ro-19-8022
This is a classic example of a promising new drug gone horribly wrong. It was originally developed as a sedative, but when screened for its ability to cause mutations in bacteria (using the Ames test, a quick way of testing to see if something causes cancer without actually having to expose an animal to it and wait to see what develops), it was found to cause mutations in the presence of light. It's chemical structure is such that it can inadvertently absorb light energy and use it to generate reactive oxygen species, which then cause oxidative damage to DNA, leading to mutations.
INC x-men!~