TO APPRECIATE the dilemmas the Food and Drug Administration (FDA) faces when it reviews new drugs targeting rare diseases, consider the recent rollercoaster ride of Sarepta Therapeutics, a biotech firm. In 2023 the FDA granted accelerated approval for its gene therapy for Duchenne muscular dystrophy, a rare genetic disorder that typically causes death in early adulthood. Peter Marks, director of the FDA’s biologics centre at the time, approved the treatment despite reviewers’ concerns about limited clinical data. Some hailed that as proof the FDA could be nimble in cases where sufferers had few options. The treatment costs an eye-watering $3.2m per patient, so the drug’s approval delivered Sarepta a major payday. But in the past four months three patients on Sarepta’s treatments have died from acute liver failure. On July 18th the FDA asked the firm to suspend distribution. Within days its stock lost nearly half of its value (see chart 1).