In addition to genetic variation, the OPRM1 gene also has substantial structural variation. Alternative splicing of 15 known exons produces at least 23 previously described splice variants, with 16 of these variants potentially translated into protein products (http://www.ensembl.org). Despite the large number of total exons, individual splice variants each contain only 3-5 exons. The 3′ UTR of OPRM1 is also known to vary in size, with some isoforms in both mice and humans known to have UTRs greater than10 kb in length [8, 9]. Given the known roles of 3′ UTRs in regulating transcript expression through miRNA binding and other mechanisms, the variable UTR length in OPRM1 may help regulate expression levels of the different isoforms [10].
