GNAS and fibrous dysplasia: Fibrous dysplasia (FD) of the bone (OMIM #174800, ORPHA: 249) is a rare genetic bone disorder caused by postzygotic activating variants (primarily at the R201 [1] and more rarely at Q227 codons [2]) in GNAS, which encodes the α subunit of stimulatory G protein (Gαs) [3–5].