Sperm protein required for fusion of sperm with the egg membrane during fertilization. Highly expressed in testis (PubMed:24275887, PubMed:32393636). Minor expression also detected in epididymis, seminal vesicle and ovary (PubMed:32393636). Predominantly expressed in testicular germ cells during spermiogenesis (PubMed:24275887). Most abundant in round spermatids and detected at lower levels in elongating spermatids (PubMed:24275887). Expression is first detected on postnatal day 21. Male infertility (PubMed:24275887, PubMed:32393636). Testis appearance, size and weight are normal and there is no effect on sperm morphology or motility but there is a failure of sperm-egg interaction and fusion (PubMed:24275887, PubMed:32393636). Spermatozoa undergo normal acrosomal reaction and can penetrate the zona pellucida but accumulate in the perivitelline space as they are unable to fuse with the egg plasma membrane to complete fertilization (PubMed:24275887, PubMed:32393636). No effect on amount or localization of sperm-egg fusion protein IZUMO1 (PubMed:32393636). Belongs to the SPACA6 family.