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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, and light skin and hair. Most 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 the maternal chromosome 15 from the mother and lacks the paternal copy. As parts of the chromosome from the mother are turned off through imprinting, they end up with no working copies of certain genes. PWS is not generally inherited, but rather the genetic changes happen during the formation of the egg, sperm, or in early development. No risk factors are known for the disorder. Those who have one child with PWS have less than a 1% chance of the next child being affected. A similar mechanism occurs in Angelman syndrome, except the defective chromosome 15 is from the mother, or two copies are from the father. Prader–Willi syndrome has no cure. Treatment may improve outcomes, especially if carried out early. In newborns, feeding difficulties may be supported with feeding tubes. Strict food supervision is typically required, starting around the age of three, in combination with an exercise program. Growth hormone therapy also improves outcomes. Counseling and medications may help with some behavioral problems. Group homes are often necessary in adulthood. PWS affects between 1 in 10,000 to 30,000 people worldwide. The condition is named after Swiss physicians Andrea Prader and Heinrich Willi who, together with Alexis Labhart, described it in detail in 1956. An earlier description was made in 1887 by British physician John Langdon Down.
