Prader–Willi syndrome is a rare but major genetic condition that involves being overweight, substandard sexual development and low intelligence. This problem also puts those affected under risk for diabetes mellitus. People with Prader-Willi syndrome experience constant hunger, despite eating much more than average, which can cause life-threatening obesity. It was first observed in 1956. Prader-Willi syndrome is usually diagnosed with a genetic test. There is no cure for Prader-Willi syndrome. It can be treated with behavioural therapy.