Endomorphin-1 (EM-1) is an edogenous opioid peptide and one of the two endomorphins (EM-1 and EM-2). EM-1 is a potent analgesic having high affinity (Ki = 360 pM) and selectivity for mu-opioid receptors. EM-1 produces analgesia in animals as equipotent as morphine.H-Tyr-Pro-Trp-Phe-NH2 or H-YPWF-NH2