The High Mobility Group 1 protein (HMG1) is an abundant and highly conserved chromosomal protein. Mouse HMG1 is encoded by the Hmg1 gene, containing four introns, but the murine genome contains many related sequences that are mostly retrotransposed pseudogenes. By using an interspecific cross, we have mapped the functional Hmg1 gene on mouse Chromosome (Chr) 5 and seven Hmg1-related sequences on Chrs 6, 8, 17, 18, and X.