PKDREJ (polycystin family receptor for egg jelly) is a testis-specific transmembrane protein localized to the sperm plasma membrane, particularly in the acrosomal region 1. As a member of the polycystin family, PKDREJ functions as a sperm receptor homologous to the sea urchin receptor for egg jelly 2, playing a central role in mammalian fertilization. The protein modulates G-protein signaling and associates with polycystin-2 family ion channels 3, regulating sperm transport and the timing of zona pellucida-induced acrosome reaction 4. PKDREJ exhibits evidence of positive selection among primates and within humans, suggesting adaptive evolution driven by sperm-egg coevolution 5. In rodents, evolutionary rates of Pkdrej correlate with testes mass relative to body mass, indicating post-copulatory sexual selection pressure 4. Population-level analysis reveals that PKDREJ alleles previously maintained by long-term balancing selection in African populations have experienced recent positive selection in Eurasian populations 6. These findings establish PKDREJ as a functionally important fertilization protein whose evolution is shaped by both sperm competition and environmental adaptation, with potential implications for human fertility 5.