RNASE9 is an epididymis-specific, secreted protein belonging to the ribonuclease A superfamily that plays a role in sperm maturation despite lacking ribonuclease activity. Located on chromosome 14.2, RNASE9 is expressed throughout various tissues with higher expression in adult males 1. Within the epididymis, the protein localizes to epithelial cells and associates with spermatozoa on the post-equatorial head region and neck 12. Functionally, RNASE9 exhibits no detectable ribonucleolytic activity against standard substrates 12, but demonstrates antibacterial activity against E. coli in a concentration-dependent manner, suggesting a role in host defense of the male reproductive tract 2. In vivo studies using Rnase9 knockout mice reveal that while the protein is dispensable for fertility and spermatogenesis, its absence during epididymal transit results in impaired sperm maturation, characterized by early motility defects and decreased phosphorylation of protein kinase A substrates during capacitation 3. Additionally, RNASE9 gene expression shows dose-dependent correlation with inflammatory responses in pulmonary injury models, suggesting broader involvement in defense mechanisms 4. Overall, RNASE9 functions as a non-enzymatic defensive protein contributing to normal sperm maturation.