Notable exceptions include mutations in genes encoding proteoglycan core proteins or enzymes involved in their biosynthesis or assembly, as models for such cartilage diseases as Keipert syndrome (Glypican 4, GPC4; discussed in section 1a), osteosclerotic bone dysplasia (FAM20C), and hereditary multiple exostoses (Exostosin 2, EXT2). The gene discussed is GPC4; the disease is hereditary multiple exostoses.