Endomorphin-1 (EM-1) (amino acid sequence Tyr-Pro-Trp-Phe-NH2) is an endogenous opioid peptide and one of the two endomorphins. It is a high affinity, highly selective agonist of the μ-opioid receptor, and along with endomorphin-2 (EM-2), has been proposed to be the actual endogenous ligand of the μ-receptor. EM-1 produces analgesia in animals and is equipotent with morphine in this regard. The gene encoding for EM-1 has not yet been identified, and it has been suggested that endomorphins could be synthesized by an enzymatic, non-ribosomal mechanism.