Prader–Willi syndrome (PWS) is a genetic disorder caused by a loss of function of specific genes on chromosome 15. In newborns, symptoms include weak muscles, poor feeding, and slow development. Beginning in childhood, those affected become constantly hungry, which often leads to obesity and type 2 diabetes. Mild to moderate intellectual impairment and behavioral problems are also typical of the disorder. Often, affected individuals have a narrow forehead, small hands and feet, short height, light skin, and hair, and are unable to have children.

About 74% of cases occur when part of the father's chromosome 15 is deleted.  In another 25% of cases, the affected person has two copies of chromosome 15 from their mother and none from their father. As parts of the chromosome from the mother are turned off through imprinting, they end up with no working copies of certain genes.