Eppin, an epididymal protease inhibitor, belongs to the WFDC family and to the telomeric cluster. The gene encoding the protein is localized to chromosome 20q12-q13 in centromeric and telomeric clusters. It expresses three mRNAs encoding two isoforms of a cystine-rich protein that contains Kunitz-type and WAP-type (four disulfide core) protease inhibitor consensus sequences. The mouse gene lies in a cluster of putative Eppin-like genes on mouse chromosome 2. Following ejaculation, Eppin is bound to semenogelin in seminal plasma and on human spermatozoa. This complex of Eppin and Semenogelin can provide antimicrobial activity for spermatozoa. It can also provide for the preparation and survival of spermatozoa for fertility in the female reproductive tract. Eppin, which is a secreted protein, is expressed in epididymis and testis.