May be involved in the fusion of the spermatozoa with the oocyte during fertilization. Interacts with C3b. Interacts with C4b. Interacts with moesin/MSN. Inner acrosomal membrane of spermatozoa. Additional isoforms seem to exist. Present only in testis (at protein level). Not expressed until 29 dpc. Expressed in parallel with synthesis of spermatids. May be O-glycosylated. N-glycosylated. Mice have normal testis and fertile sperm. Lacks transmembrane domain, probably secreted.