IGF1 and Noonan syndrome: Finally, heterozygous gain-of-function mutations of PTPN11, which encodes the protein phosphatase SHP-2, are one genetic etiology of Noonan syndrome that has been suggested to function in altering the GH–IGF-1 axis [36-38]; however, the data demonstrating a distinction with other genetic etiologies of Noonan syndrome is not entirely conclusive [39,40].