Figure 3: The genetic complexity of μ-receptors.

Although only a single μ-receptor encoding gene exists in humans and rodents (the latter shown above), alternative splicing of the OPRM1 pre-mRNA gives rise to a plethora of receptor variants that exhibit distinct biochemical properties. All 7TM variants contain a consensus sequence built from exons 1, 2 and 3, whereas 6 TM variants replace exon 1 with exon 11. Note that the exon nomenclature does not reflect their chromosomal locus but rather their timepoint of discovery.