Studies have suggested association of a number of gene variants with heroin addiction or combined drug addiction. These include variants in genes encoding opioid, dopamine, serotonin, and GABA receptors and also cholinergic muscarinic 2, ACTH, and cannabinoid 1 receptors. In addition, association was suggested for catechol-O-methyltransferase (COMT), period circadian protein 3, proenkephalin, proopiomelanocortin, tryptophan hydroxylase 2, and brain-derived neurotrophic factor (Kreek et al., 2005, Cheng et al., 2005, Loh et al., 2007, Nielsen et al., 2008, Proudnikov et al., 2008, Proudnikov et al., 2006, Szilagyi et al., 2005, Xu et al., 2004, Zou et al., 2007, Zuo et al., 2007)}. We have recently conducted a candidate gene association study in European Americans (EA), in which we detected associations with SNPs in OPRM1, OPRK1 OPRD1, GAL, HTR3B and CSNK1E, with severe heroin addiction (Levran et al., 2008)
