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:28554943). May be involved in the regulation of serine protease-dependent germ cell apoptosis (By similarity). It also inhibits trypsin. {ECO:0000250|UniProtKB:Q8BMY7, ECO:0000269|PubMed:19422058, ECO:0000269|PubMed:28554943}. This is the function of Ensembl gene identifier ENSG00000128040 (SPINK2, serine peptidase inhibitor Kazal type 2).