As a strong inhibitor of acrosin, it is required for normal spermiogenesis. It probably hinders premature activation of proacrosin and other proteases, thus preventing the cascade of events leading to spermiogenesis defects (PubMed:21705336, PubMed:28554943). May be involved in the regulation of serine protease-dependent germ cell apoptosis (PubMed:21705336). It also inhibits trypsin (PubMed:21705336). Expressed in sperm (at protein level). Expressed in testis but not in ovary, brain, heart, kidney or lung. Within testis, expressed in epididymis and germ cells. First expressed at 16 days postpartum (dpp). Level increases until 20 dpp and is maintained into adulthood (at protein level). Knockout male mice are completely infertile, whereas no reproductive defects are observed in females. Spermatozoa are completely absent from caudal epididymis, which only contains round cells likely corresponding to round spermatids and multinucleated cells. Seminiferous tubules contain germ cells up to the early round-spermatid stage but condensed and elongated spermatids and mature spermatozoa are completely absent. Round spermatids do not contain an acrosomal vesicle. On the 2D-gel the determined pI of this protein is: 5, its MW is: 8 kDa.