Multiple exostoses is a hereditary disease characterized by multiple osteocartilagenous tumors, of which the histological structures are similar to those of normal epiphyses. Genetic linkage has identified three different loci for this disease: EXT1 on 8q, EXT2 on 11p, and EXT3 on 19p. The EXT1 and EXT2 genes were recently isolated and mutation analyses have been performed in a number of patients with different ethnic backgrounds. The data indicate that mutations of these genes occurred in broad regions of each gene, and the loss-of-function mutations were predominant, although there were some missense mutations that may create functionally defective protein. Tumor cells were shown to be homozygous for the mutant allele, which is consistent with the concept of these genes as tumor suppressor genes. Recent progress for the functional analyses has disclosed that these genes encode the protein with glycosyltransferase activity and regulate the diffusion of Hedgehog protein, which is the key molecule for the skeletal development. Further analyses of these genes may provide us with the knowledge for the development of epiphyses, and may open the new research field for the regeneration of epiphyses.