Has antibacterial activity against the Gram-negative bacteria E.coli. May play a role in sperm maturation and fertilization. Probably secreted into the epididymis lumen, where it localizes to the outer surface of sperm. Specifically localizes to the postacrosomal and tail regions of sperm. Expressed in epididymis, where it localizes to the proximal caput and also part of the midcaput. Not detected in other tissues tested. Up-regulated by testicular factors. However, does not seem to be directly regulated by androgens or estrogen. Belongs to the cystatin family.