GNAS and McCune-Albright syndrome: McCune-Albright syndrome (MAS) is characterized by the triad of precocious puberty, café au lait pigmentation, and polyostotic fibrous dysplasia (FD) of bone, and is caused by post-zygotic somatic mutations—R201H or R201C—in the guanine nucleotide binding protein, alpha stimulating (GNAS) gene.