Prader-Willi syndrome (PWS) is caused by a rare birth defect centered on chromosome 15. Characteristics of the syndrome include developmental delays, mental retardation, behavioral problems, and insatiable appetite leading to obesity. Affected individuals also experience incomplete sexual development, poor muscle tone, and short stature as adults.

PWS occurs in 1 in 12,000 to 15,000 births and is regarded as the most common genetic cause of obesity. It affects both genders and all races. Although PWS arises from a genetic defect, it is not an inherited condition--it is a birth defect. The defect occurs spontaneously and specifically involves chromosome 15.

A person normally inherits one copy of chromosome 15 from each parent. In PWS cases, the copy from the father either lacks a specific segment of DNA (70-75% of cases) or is missing altogether (25-30% of cases). If the father's chromosome 15 is absent, a person with PWS has two copies of the mother's...