The HMGIY non-histone proteins play important roles as architectural transcription factors that regulate gene transcription in mammalian cells and also act as host-supplied cofactors necessary for retroviral integration. The genes coding for the HMGIY proteins are proto-oncogenes, and their aberrant or over-expression is correlated with both neoplastic transformation and metastatic progression in a wide variety of tumors. Here, we report the first complete sequence of the murine Hmgiy (a.k.a. Hmga1) gene and provide a detailed comparison of this with the sequence and organization of the human HMGIY gene, including an analysis of its promoter region with the previously unreported 5' upstream region of the human gene. These analyses reveal a remarkable degree of overall sequence conservation in both the protein coding and promoter regions of the murine and human genes, including conservation of the c-Myc binding site that has been demonstrated to regulate murine Hmgiy transcription (Wood et al., 2000. Mol. Cell. Biol. 20, 5490-5502). The promoters of both genes contain other conserved transcription factor binding sites that may also represent important cis-regulatory elements. Two exons present in the 5' untranslated region of the human gene, however, are missing from the murine gene, suggesting that these two closely related mammalian species regulate transcription of their Hmgiy genes in an individualistic manner.