Hereditary multiple exostoses (EXT) is an autosomal dominant disorder characterized by multiple cartilage-capped outgrowths from the epiphyses of long bones. In some cases, these osteochondromas progress to malignant chondrosarcomas. Alterations in at least three genes (EXT1, EXT2, and EXT3) can cause this disorder. Two of these have been isolated (EXT1 andEXT2) and encode related members of a putative tumor suppressor family. We report here the genomic structure of the humanEXT2 gene consisting of 14 exons (plus 2 alternative exons) covering an estimated 108 kb of chromosome 11p11–13. We have derived the DNA sequences at all exon/intron boundaries throughout this gene—information that is important for the detailed study of mutations in EXT2. We have also characterized the mouse EXT2cDNA and have mapped the mouse locus to chromosome 2 betweenD2Mit15 and Pax6. This mouse homolog should enable transgenic knockout experiments to be initiated to further elucidate gene function. Interestingly, sequence comparisons reveal that the human and mouse EXT genes have at least two homologs in the invertebrate Caenorhabditis elegans, indicating that they do not function exclusively as regulators of bone growth. This observation opens the way for a functional analysis of these genes in nematodes and other lower organisms.[The sequence data described in this paper have been submitted to the GenBank data library under accession nos. U67353–U67368 and U67837.]