The HMGI proteins, a subfamily of the high mobility group (HMG) proteins, play an im-portant role in the regulation of gene expression. They bind to AT-rich DNA sequences and alter DNA conformation to modulate the binding affinity of transcription factors to their cognate sites. They are expressed almost exclusively during embryogenesis and Hmgi-c null mice have the mouse pygmy phenotype. Studies have revealed the disruption of HMGI family genes in a variety of mesenchymal-derived benign tumors. Therefore, the HMGI genes function in the coordination of cell proliferation and differentiation during mammalian development.