MOPRs, belonging to the seven transmembrane receptor (7TMR) family or G protein-coupled receptor (GPCR) family, mediate pharmacological effects of morphine and other μ-preferring compounds. Activation of the MOPRs produces analgesia, reward, mood changes, sedation, respiratory depression, immuno-suppression, decreased gastrointestinal motility, increased locomotor activity, tolerance and dependence [for reviews, see [1,2]]. Bergen et al. [3] and Bond et al. [4] first reported the existence of A118G SNP in the coding region in the exon 1 of the hMOPR gene (OPRM1). This SNP has been found to have the highest overall allelic frequency of all the OPRM1 coding region variants. The G118 allele frequency varies widely across populations: 1% to 3% in African Americans, 10-14% in both Caucasians and Hispanics, 35-49% in Asians and 8-21% in other populations [reviewed in [5]]. Four clinical studies conducted on East Asians showed that subjects homozygous for G118 needed higher morphine doses to attain adequate pain control following surgery than those of homozygous for A118 [6-9]. In addition, subjects of G/G or A/G genotype have better treatment outcomes for nicotine and alcohol abuse [10-12] and higher propensity for drug addiction [13-16][reviewed in [17]].
