Secreted proximal epididymal protein required for post-testicular sperm maturation and male fertility. May be involved in sperm adhesion to the egg zona pellucida. Does not have ribonuclease activity. Male-specific expression in proximal caput of the epididymis (at protein level). Expressed in mature epididymis. The N-terminus is blocked. Glycosylated (By similarity). Mice are viable, anatomically normal and reach adulthood but display impaired fertility consequent to an inability of the spermatozoa to ascend the uterotubal junction (UTJ) canal of the female reproductive tract and gain the site of fertilization. Spermatozoa fail to establish a strong association with either epididymal epithelial cells, the zona pellucida of oocytes or oviductal epithelial cells, yet they are capable of fertilizing eggs in vitro. Belongs to the pancreatic ribonuclease family. Extended N-terminus.