OPRM1 (OMIM: 600018) is a natural candidate gene for studies of opioid dependence (OD) as it encodes the µ-opioid receptor, the primary target of opioids. A common polymorphism in exon 1 of OPRM1 (rs1799971 or 118A>G) results in an Asn40Asp amino-acid change. Initial reports suggested the 118G variant could increase the receptor’s affinity for β-endorphin, its endogenous ligand (Bond et al., 1998). Though this was not confirmed in subsequent work (Befort et al., 2001; Beyer et al., 2004), other studies indicate that the 118G allele might still alter OPRM1 function, either by lowering OPRM1 expression (Beyer et al., 2004) or through a deleterious effect on mRNA and protein yield (Drakenberg et al., 2006; Zhang et al., 2005).
