Plays a role in sperm-oocyte fusion process during fertilization. Localized to the equatorial segment of the sperm head. Testis-specific. Deficient mice have no overt developmental abnormalities. Furthermore, no deleterious effects on testicular histology and sperm morphology are observed. However male mice are severely subfertile, their spermatozoa show impaired ability to fuse with the oocytes. Truncated N-terminus. Truncated N-terminus.