Linkage analyses aimed at identifying the underlying cause of HMO led to the identification of probable disease loci on chromosomes 8q and 11p and the homologous genes EXT1 and EXT2 (OMIM IDs 608177 and 608210) were later characterized at these locations (Ahn et al., 1995; Stickens et al., 1996). Here, EXT1 is linked to Multiple osteochondromas.