Definition of Noonan syndrome:

Noonan syndrome is a genetic disorder that causes abnormal development of multiple parts of the body. It used to be called Turner-like syndrome because certain symptoms (webbing of neck and abnormally shaped chest) resembled those seen in Turner syndrome.

Causes, incidence, and risk factors:

Defects in the KRAS and PTPN11 genes cause Noonan syndrome. About half of those affected by Noonan syndrome have a PTPN11 mutation. Persons with a defect in the KRAS gene have a severe or atypical form of Noonan syndrome. Problems with these genes cause certain proteins involved in growth and development to become overactive.

Noonan syndrome is inherited, which means it is passed down through families. It is an autosomal dominant condition. However, the fact that some children do not have a parent with Noonan syndrome likely reflects that some cases may occur due to new mutation in a gene.

The disease occurs in approximately 1 in 1,000 to 2,500 children.