The difference between social and solitary worms is caused by a single amino acid substitution in a single gene, a member of a large family of genes involved in signaling between neurons. This gene, npr-1, encodes a neuropeptide receptor. Neuropeptides have long been appreciated for their roles in coordinating behaviors across networks of neurons. For example, a neuropeptide hormone of the marine snail Aplysia stimulates a complex set of movements and behavior patterns associated with a single behavior, egg laying. Mammalian neuropeptides have been implicated in feeding behavior, sleep, pain, and many other behaviors and physiological processes. The existence of a mutation in the neuropeptide receptor that alters social behavior suggests that this kind of signaling molecule is important both for generating the behavior and for generating the variation between individuals.