Serine protease inhibitor that plays an essential role in male reproduction and fertility. Modulates the hydrolysis of SEMG1 by KLK3/PSA (a serine protease), provides antimicrobial protection for spermatozoa in the ejaculate coagulum, and binds SEMG1 thereby inhibiting sperm motility. Monomer. Homodimer. Homomultimers. Interacts with SEMG1 (via 164-283 AA). Interacts with LTF. Found in a complex with LTF, CLU, EPPIN and SEMG1. Bound to the surface of testicular and on the head and tail of ejaculate spermatozoa. In testis, expressed and secreted by Sertoli cells, appearing on the surface of testicular and ejaculate spermatozoa. Expressed in the spermatogonia and the earliest preleptotene spermatocytes. In the epididymis, is expressed and secreted by epithelial cells and covers the surface of epididymal spermatozoa and ciliated epithelial cells (at protein level). Expressed specifically in epididymis and testis. Isoform 2 is expressed only in the epididymis. Weak expression is detected in myoid cells as well as spermatogenic cells. The BPTI/Kunitz inhibitor domain is required for elastase inhibitory activity. BPTI/Kunitz inhibitor and WAP domains are involved in the protein antibacterial activity. Might be used as a target for male contraception. Lacks a cleavable signal sequence. Based on a readthrough transcript which may produce a EPPIN-WFDC6 fusion protein.