Prader–Willi syndrome is a rare genetic disorder. It causes obesity, slow sexual development and low intelligence. People with Prader–Willi syndrome also have a higher chance of getting diabetes mellitus, and cannot have children. They are hungry very often, and eat much more than other people. It was first described in 1956. Prader-Willi syndrome is usually diagnosed with a genetic test. Prader-Willi syndrome has no cure. It can be treated with behavioural therapy.