We describe two cases of extraskeletal osteochondroma in which chromosome bands 12q14~ 15 were visibly rearranged through a pericentric inv (12). Molecular analysis of the first tumor showed that both transcript 1 (NM_003483) and transcript 2 (NM_003484) of HMGA2 were expressed. In the second tumor, the inv (12) detected by karyotyping had resulted in an HMGA2-SOX5 fusion transcript in which exons 1-3 of HMGA2 were fused with a sequence from intron 1 of SOX5. The observed pattern is similar to rearrangements of HMGA2 found in several other benign mesenchymal tumors, ie, disruption of the HMGA2 locus leaves intact exons 1-3 which encode the AT-hook domains and separates them from the 3'-terminal part of the gene. Our data therefore show that a subset of soft tissue osteochondromas shares pathogenetic involvement of HMGA2 with lipomas, leiomyomas and other benign connective tissue neoplasms.