OPRK1 and major depressive disorder: Mu opioid receptors (MORs), which belong to the opioid receptor family, are involved in diverse physiological and pathological states, and MOR agonists were found to produce analgesia, alleviate depression symptoms, and relieve stress and anxiety [13].Furthermore, genetic deletion of MOR and delta opioid receptor (DOR), but not kappa opioid receptor (KOR)as shown to increase anxiety symptoms and depressive-like behavior [14].