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 holoprosencephaly and the nevoid basal cell carcinoma syndrome, the former being caused by sonic hedgehog (SHH) mutations and the latter being caused by patched mutations (PTCH).