PRDM9 and Infertility: The proper allelic combination of Prdm9 and Hstx2 genes is necessary but not sufficient to govern HS completely because less than 10% instead of the expected 25% of (PWD x B6) x B6 male backcross progeny replicated the infertility of male PB6F1 hybrids (Dzur-Gejdosova et al., 2012).