The A118G single nucleotide polymorphism (SNP) of the human µ opioid receptor gene (OPRM1) is the most common SNP in the coding region of the MOPR (Bergen, Kokoszka, Peterson et al., 1997; Bond, LaForge, Tian et al., 1998). This SNP results in a change of amino acid at position 40 from asparagine to aspartate (N40D) in the N-terminal domain, which removes one of five potential N-linked glycosylation sites of the receptor (Bergen et al., 1997; Bond et al., 1998). Recently we reported that this SNP reduced the N-glycosylation and stability of the MOPR protein (Huang, Chen, Mague et al., 2012). The A118G allele frequency varies among ethnic groups: 1–3% in African Americans, 10–14% in both Caucasians and Hispanics, 35–49% in Asians and 8–21% in other populations [reviewed in (Kreek, Bart, Lilly et al., 2005; Mague and Blendy, 2010)]. Clinical studies revealed that among alcohol dependent patients treated with naltrexone, those with one or two copies of the G118 allele had better outcome (Oslin, Berrettini, Kranzler et al., 2003; Kim, Kim, Choi et al., 2009). Among individuals using transdermal nicotine for
