The mouse cDNA encoding the major protein of the outer dense fibers in sperm tails was isolated by reverse transcription of testicular RNA and amplification with sequence-specific primers. Sequencing of a genomic clone obtained by inverse PCR yielded the 5' untranslated region. The transcription starting point was verified by primer extension. The putative proteins encoded by Odf1 in mouse and by ODF1 in rat and man are very similar. A total of 15 amino acids in the C-terminal region were deleted in the mouse protein, compared with the rat protein. Through in situ hybridization to metaphase chromosomes, the Odf1 gene was localized to mouse chromosome 15 region B2-C. The chromosomal localization of the Odf1 gene extends the hitherto known linkage group consisting of MYC (Myc), PVT1 (Pvt1), GPT (Gpt), and TG (Tg) common to human chromosome 8 and mouse chromosome 15 in the proximal direction of both chromosomes. The linkage group now extends from band q24 to band q22 of human chromosome 8 and from region D2-E to region B2-C of mouse chromosome 15.