OPRPN encodes opiorphin prepropeptide, an endogenous inhibitor of neprilysin and aminopeptidase N involved in neuropeptide degradation. UniProt annotations document that opiorphin inhibits breakdown of substance P, enkephalins, and bradykinin with IC50 values of 29-65 μM, and demonstrates potent analgesic effects in rats. GO terms indicate peptidase inhibitor activity and roles in pain perception regulation. Based on limited published evidence, OPRPN expression is induced by ionizing radiation in head and neck squamous cell carcinoma cells 1, where elevated OPRPN may associate with improved clinical outcomes, suggesting potential protective functions in radiotherapy response 1. OPRPN is expressed in salivary glands, though specific functional roles in salivary biology remain understudied.