Two genes, EXT1 (OMIM: 608177) and EXT2 (OMIM: 608210), which are located respectively at 8q24.11 and 11p11.2, have been identified to cause HMO [3, 4], with an estimated penetrance of 100% [5] and variable expressivity, especially in females [2]. Here, EXT1 is linked to hereditary multiple osteochondromas.