At the OPRM1 locus, linkage disequilibrium (LD) exists between OPRM1 A118G and other markers 31. We therefore generated two humanized mouse lines, where the OPRM1 exon 1 was replaced by the corresponding human sequence. One of the lines, h/mOPRM-118AA, is homozygous for the major human 118A allele. For the other (h/mOPRM1–118GG), site directed mutagenesis was used to introduce a G in position 118. Thus, the lines are genetically identical, with the exception of the A → G substitution (Figure 2A–C). The lines are on a C57BL/6 background, where robust voluntary alcohol consumption is observed 32.
