The SPO11/TOPVIA family includes proteins from archaebacteria and eukaryotes. The protein member from the archaebacterium Sulfulobus shibatae is the catalytic subunit of TopoVI DNA topoisomerase. In Saccharomyces cerevisiae, Schizosaccharomyces pombe, Caenorhabditis elegans and Drosophila melanogaster, SPO11 is required for meiotic recombination, suggesting a conserved mechanism for the initiation step of this process. Indeed, S. cerevisiae SPO11 has been shown to be directly involved in the formation of meiotic DNA double-strand breaks that initiate meiotic recombination. Here, we report the identification of a Mus musculus Spo11 cDNA, which encodes a protein closely related to all members of the SPO11/TOPVIA family. cDNAs resulting from alternative splicing were detected, suggesting that there are potential variants of the mouse SPO11 protein. By RNA-blotting analysis, expression of the mouse Spo11 gene was detected only in the testis, in agreement with its predicted function in the initiation of meiotic recombination. We mapped the mouse Spo11 gene to chromosome 2, band H2-H4.