The hierarchical clustering of GO categories ( Figure 2 ) reveals an unexpected similarity between the sets of PSGs involved in fertilization and cytolysis, and some similarity of both sets with immune-related PSGs. This association of immunity, fertilization, and cytolysis is driven by a group of genes that participate in sperm-egg interaction, but also have immune-related functions and destroy pathogens by cytolysis. Interestingly, PSGs with roles in both reproduction and immunity are often also related to cancer, and it has been hypothesized that most cancer genes under positive selection have been subject to antagonistic co-evolution, with lineage-specific variations in dynamics and strength [5, 37] . Several PSGs identified here are associated with both FAS/p53 apoptosis and cancer (Da Fonseca et al., in prep.), such as the protein p53, which also regulates maternal reproduction [38] ; the cell adhesion gene ADAM2 (P = 2.9610 26 ), which is integral to fertilization [39] ; and the related genes ADAM15 (P = 5.4610 24 ) and ADAM29 (P = 3.4610 24 ), which are strong candidates for cancer evolution driven by sexual conflict. In addition, the testes development-related gene CCDC54 (P = 3.3610 24 ) is currently a target of cancer immunotherapy research [40] .