Noonan syndrome (NS1, OMIM 163950), caused by mutation and activation of the genes involved in the RAS-MAPK pathway, including PTPN11, SOS1, KRAS, NRAS, RAF1, BRAF, RIT1, and LZTR1, is a common developmental disorder with an autosomal dominant inheritance. Here, PTPN11 is linked to Noonan syndrome.