GNAS and McCune-Albright syndrome: Activating missense mutations in the guanine nucleotide-binding protein α-subunit (GNAS) gene, which encodes the stimulatory α subunit of the G-protein (Gsα), resulting in a change at the Arg 201 codon from arginine to cysteine (Arg-to-Cys, R201C) or arginine to histidine (Arg-to-His, R201H) have been identified in both the monostotic and polyostotic forms of FD, as well as in McCune-Albright syndrome (5–7).