Prader-Willi syndrome is characterized by severe infantile hypotonia with poor 
suck and failure to thrive; hypogonadism causing genital hypoplasia and pubertal 
insufficiency; characteristic facial features; early-childhood onset obesity and 
hyperphagia; developmental delay/mild intellectual disability; short stature; 
and a distinctive behavioral phenotype. Sleep abnormalities and scoliosis are 
common. Growth hormone insufficiency is frequent, and replacement therapy 
provides improvement in growth, body composition, and physical attributes. 
Management is otherwise largely supportive. Consensus clinical diagnostic 
criteria exist, but diagnosis should be confirmed through genetic testing. 
Prader-Willi syndrome is due to absence of paternally expressed imprinted genes 
at 15q11.2-q13 through paternal deletion of this region (65-75% of individuals), 
maternal uniparental disomy 15 (20-30%), or an imprinting defect (1-3%). 
Parent-specific DNA methylation analysis will detect >99% of individuals. 
However, additional genetic studies are necessary to identify the molecular 
class. There are multiple imprinted genes in this region, the loss of which 
contribute to the complete phenotype of Prader-Willi syndrome. However, absence 
of a small nucleolar organizing RNA gene, SNORD116, seems to reproduce many of 
the clinical features. Sibling recurrence risk is typically <1%, but higher 
risks may pertain in certain cases. Prenatal diagnosis is available.