The high-mobility group (HMG) proteins are a family of nonhistone chromatin-associated proteins that have an important role in regulating chromatin architecture, as well as in regulating gene expression. The gene encoding one HMG protein, HMGI-C, is frequently rearranged or overexpressed by chromosomal translocations in common benign mesenchymal tumors including lipomas, leiomyomas, fibroadenomas, pleomorphic adenomas, aggressive angiomyxomas, and pulmonary hamartomas. The HMGI-C translocations involve many different translocation partners and are remarkable for their low risk of progression to malignancy. Recently, another member of this subfamily, HMGI(Y), has also been shown to be rearranged in lipomas and pulmonary chondroid hamartomas. Given the high frequency of these benign mesenchymal tumors, it is likely that translocations involving the HMGI subfamily are one of the most common chromosomal rearrangements in human neoplasia. The HMG proteins are reviewed with an emphasis on the HMGI subfamily, and the potential role of these proteins in human tumorigenesis is discussed.