GNAS and bone disorder: FD is a potentially crippling skeletal disease caused by post-zygotic activating mutations of the alpha subunit of the stimulatory G protein encoded by the GNAS gene (GNAS complex locus; GNAS, OMIM *139320) that can occur either isolated or in association with non-skeletal disorders (mainly hyper-functional endocrinopathies and skin hyperpigmented lesions) in the MAS (for review see [5]).