Sperm protein required for fusion of sperm with the egg membrane during fertilization. Detected in testicular germ cells and spermatozoa (at protein level). Abundantly expressed in testis. Detected in the testis after postnatal day 28. Male infertility (PubMed:32393636). Sperm are morphologically normal, exhibit normal motility and can penetrate the zona pellucida but are unable to fuse with the egg membrane (PubMed:32393636). No effect on amount or localization of sperm-egg fusion protein IZUMO1 (PubMed:32393636).