EXT1 and Osteochondroma: EXT1 and EXT2 are Golgi-resident glycosyltransferases participating in the biosynthesis of HS.238 Mutations of EXT1 and EXT2 in humans cause hereditary multiple exostoses (MIM #133700, #133701), a human autosomal skeletal disorder characterized by the formation of cartilage-capped bony growths (osteochondroma) at the ends of the bones, due to excessive BMP signaling.238 Mouse models with CKO of Ext1 in cartilage tissue develop osteochondroma and enhanced chondrocyte hypertrophy due to increased BMP-SMAD activity.239–241