Secreted proximal epididymal protein required for post-testicular sperm maturation and male fertility. May be involved in sperm adhesion to the egg zona pellucida (By similarity). Does not have ribonuclease activity. Male-specific expression in proximal caput of the epididymis. The N-terminus is blocked. Glycosylated. Represents 34% of the proteins secreted by the epididymis and 89% of the proteins secreted by the proximal caput. Belongs to the pancreatic ribonuclease family.