From OMIM
Multiple hereditary exostoses (EXT) is an autosomal dominant disorder characterized by multiple projections of bone capped by cartilage, most numerous in the metaphyses of long bones, but also occurring on the diaphyses of long bones. Flat bones, vertebrae, and the ribs may also be affected, but the skull is usually not involved. Deformity of the legs, forearms (resembling Madelung deformity), and hands is frequent (Peterson, 1989). Two conditions in which multiple exostoses occur are metachondromatosis (156250) and the Langer-Giedion syndrome (LGS; 150230); the latter condition is also known as trichorhinophalangeal syndrome type II. Furthermore, exostosis-like lesions occur with fibrodysplasia ossificans progressiva (FOP; 135100), occipital horn syndrome (304150), and the adult stage of hereditary hypophosphatemia (see 307800); these exostoses are located at sites of tendon and muscle attachment. A relatively rare variant of the supracondylar process, on the anteromedial surface of the distal humerus, can be confused with an exostosis; the variant is said to be present in about 1% of persons of European descent (Silverman, 1985). Genetic Heterogeneity of Multiple Exostoses Multiple exostoses type II (EXT2; 133701) is caused by mutation in the EXT2 gene (608210) on chromosome 11p11. Multiple exostoses type III (EXT3; 600209) has been mapped to a locus on chromosome 19.  http://www.omim.org/entry/133700