μ (MOR, OP3) | Pro-opiomelanocortin | β-endorphin | Tyr-Gly-Gly-Phe-Met-Thr-Ser-Glu-Lys-Ser-Gln-Thr-Pro-Leu-Val-Thr-Leu-Phe-Lys-A sn-Ala-Ile-Ile-Lys-Asn-Ala-Tyr-Lys-Lys-Gly-Glu | Mammalian brain | Li and Chung (1976) |
| β-Casein (bovine) | β-Casomorphin-5 | Tyr-Pro-Phe-Pro-Gly | Bovine milk | Blanchard et al. (1987) |
| | β-Casomorphin-7 | Tyr-Pro-Phe-Pro-Gly-Pro-Ile | | |
| | Morphiceptin | Tyr-Pro-Phe-Pro-NH2 | | |
| β-Casein (human) | β-Casomorphin-5 | Tyr-Pro-Phe-Val-Glu | Human milk | Blanchard et al. (1987) |
| | β-Casomorphin-7 | Tyr-Pro-Phe-Val-Glu-Pro-Ile | | |
| Hemoglobin | Hemorphin-4 | Tyr-Pro-Trp-Thr | Human blood | Nyberg et al. (1997); Zhao et al. (1997) |
| | Hemorphin-7 | Tyr-Pro-Trp-Thr-Gln-Arg-Phe | | |
| Unknown | Dermorphin | Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2 | Frog skin | Bozu et al. (1997) |
| Unknown | Endomorphin-1 | Tyr-Pro-Trp-Phe-NH2 | Bovine brain, human brain cortex | Hackler et al. (1997); Zadina et al. (1997) |
| | Endomorphin-2 | Tyr-Pro-Phe-Phe-NH2 | | |
| Unknown | Tyr-MIF-1 | Tyr-Pro-Leu-Gly-NH2 | Bovine brain, human brain cortex | Zadina et al. (1994) |
| | Tyr-W-MIF-1 | Tyr-Pro-Trp-Gly-NH2 | | |
δ (DOR, OP1) | Proenkephalin | [Met5]enkephalin | Tyr-Gly-Gly-Phe-Met | Mammalian brain | Hughes et al. (1975) |
| | [Leu5]enkephalin | Tyr-Gly-Gly-Phe-Leu | | |
| | | Tyr-Gly-Gly-Phe-Met-Arg-Phe | | |
| | | Tyr-Gly-Gly-Phe-Met-Arg-Gly-Leu | | |
| Unknown | Dermenkephalin | Tyr-D-Met-Phe-His-Leu-Met-AspNH2 | Frog skin | Amiche et al. (1989); Kreil et al. (1989) |
| | Deltorphin-I | Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2 | | |
| | Deltorphin-II | Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2 | | |
κ (KOR, OP2) | Prodynorphin | Dynorphin A | Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln | Mammalian brain | Chavkin et al. (1982) |
| | | Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile | | |
| | Dynorphin A(1-8) | Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Gln-Phe-Lys-Val-Val-Thr | | |
| | Dynorphin B | | | |
ORL1 (NOP1) | Pronociceptin | Nociceptin/orphanin FQ | Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln | Mammalian brain | Meunier et al. (1995); Reinscheid et al. (1995) |