EXT1 and Ollier disease: 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).