In 1967, after a baby boy suffered a botched circumcision, his family agreed to a radical treatment. On the advice of a renowned expert in gender identity and sexual reassignment at Johns Hopkins Hospital, the boy was surgically altered to live as a girl. This landmark case, initially reported to be a complete success, seemed all the more remarkable since the child had been born an identical twin: his uninjured brother, raised as a boy, provided to the experiment the perfect matched control.