Serotonin is a neurotransmitter best known for its influence on depression and sexual function. But it also plays less famous roles: serotonin-releasing neurons in the brain stem regulate our body temperature and how we breathe. Henry Krous, a professor of pathology and pediatrics at the University of California, San Diego, and his colleagues have found that a failure of this system may be responsible for sudden infant death syndrome, or SIDS.