The gene encoding SPO11 shares sequence similarity to TopoVI, a type II topoisomerase. SPO11 dimers cleave both strands of DNA. Each subunit of the dimer remains covalently attached to the 5' end of one strand of DNA via a phosphodiester linkage to a conserved tyrosine residue of SPO11. In addition to SPO11, work from budding yeast has shown a total of 7 proteins essential for double strand break formation. The mammalian ortholog of Mei4 (S. cerevisiae) as well as a mammalian-specific gene called Mei1 are essential to formation of meiotic double strand breaks.