Thalidomide is a molecule with a split personality. Notorious as the drug that caused thousands of babies to be born with grotesque deformities in the 1950s, it now has become the frontline defense for some cancer and immune-compromised patients. Until recently, researchers did not understand how this single molecule could have such extreme effects. But emerging research is showing that thalidomide acts at the molecular level in an unusual way, which could explain its Dr. Jekyll-and-Mr. Hyde effects.