GNAS and polyostotic fibrous dysplasia: Mutations in the GNAS gene result in pseudohypoparathyroidism type 1a, pseudohypoparathyroidism type 1b, pseudopseudohypoparathyroidism, progressive osseous heteroplasia, polyostotic fibrous dysplasia of bone, McCune-Albright syndrome, Albright hereditary osteodystrophy, and several tumors, although the pathogenic mechanisms remain elusive [17].