Noonan syndrome (NS) is a developmental disorder characterized by short stature, facial dysmorphia, congenital heart defects and skeletal anomalies1. Increased RAS-mitogenactivated protein kinase (MAPK) signaling due to PTPN11 and KRAS mutations cause 50 percent of NS2-6. Here, we report that 22 of 129 NS patients without PTPN11 or KRAS mutation (17 percent) have missense mutations in SOS1, which encodes a RAS-specific guanine nucleotide exchange factor (GEF). SOS1 mutations cluster at residues implicated in the maintenance of SOS1 in its autoinhibited form and ectopic expression of two NS-associated mutants induced enhanced RAS activation. The phenotype associated with SOS1 defects is distinctive, although within NS spectrum, with a high prevalence of ectodermal abnormalities but generally normal development and linear growth. Our findings implicate for the first time gain-of-function mutations in a RAS GEF in inherited disease and define a new mechanism by which upregulation of the RAS pathway can profoundly change human development.