Isocitrate dehydrogenase 1/2 (IDH1 or IDH2) mutations are present in 85% of hereditary enchondromatosis-associated disorders, Ollier disease (enchondromatosis only), Maffuci syndrome (enchondromatosis and hemangiomas) and 50% of solitary enchondromas, whereas the biallelic inactivation of the exostosin glycosyltransferase (EXT1 or EXT2) genes is observed in the majority of both sporadic and hereditary osteochondromas (Figure 1). The gene discussed is EXT2; the disease is Ollier disease.