ligand binding ability and signal transduction pathways (for figures see LaForge et al. 2000). The ligands' genes all share overall similar structure with a single intron in the coding region. The opioid system is presumed to have been formed by genome duplications early in vertebrate evolution (Cruz-Gordillo et al. 2010; Li et al. 1996). Each receptor gene also produces multiple mRNA isoforms through the use of alternative splicing, alternative promoters (OPRM1, OPRK1), alternative polyadenylation sites (OPRK1), or inclusion of non-coding exons.
