The first three disorders discussed are abnormalities of bone: too little bone in cleidocranial dysplasia caused by mutations in RUNX2; too much bone in fibrodysplasia ossificans progressiva with overexpression of BMP4; and abnormal bone in McCune-Albright syndrome and fibrous dysplasia caused by mutations in GNAS1. Disorders of the sonic hedgehog signaling network are discussed next, including...