Noonan syndrome (NS): A multifaceted genetic disorder characterized by a series of birth defects (congenital malformations) including dysmorphic (malformed) facial features, short stature after birth (postnatal growth retardation), webbing of the neck (pterygium colli), caved-in chestbone (pectus excavatum), cardiovascular problems (pulmonic stenosis and hypertrophic cardiomyopathy), bleeding tendency and, in boys, testes that do not descend normally into the scrotum (cryptorchidism).
NS is an autosomal dominant disorder that carries an elevated risk of developmental and language delay, learning disability, hearing loss, and mild mental retardation. The syndrome is relatively common, with an estimated incidence of 1 in 1,000-2,500 live births. A gene for NS has been mapped to chromosome 12q24.1. More than 50% of cases are due to missense mutations in PTPN11, a gene encoding the nonreceptor protein tyrosine phosphatase SHP-2. Noonan syndrome may, it is thought, be due to excessive SHP-2 activity.\