The opioid receptors are G-protein coupled, seven-transmembrane domain receptors for enkephalins, endorphins, and dynorphins. Three different opioid receptor subtypes (kappa , delta, and mu) were first identified by their different selectivities for various naturally occurring alkaloid opioid ligands, and subsequently confirmed by molecular cloning. The amino acid sequences of the opioid receptor subtypes are 70% homologous, and are similar to somatostatin receptors (SSTRs) showing 40 % homology with SSTR1. G-protein binding is thought to occur at the third intracellular loop of the opioid receptors, which is also the location of consensus sequences for phosphorylation of the receptor. Interestingly, the genes encoding the specific receptor subtypes are found on different chromosomes in both the human and mouse genomes.