2012). Though findings from in vitro studies have been mixed (for a review, see Ray et al., 2012), the amino acid change brought on by the presence of the minor (Asp) G allele has been shown to triple the mu-opioid receptor’s binding affinity for β-endorphin and reduce cell-surface receptor binding site availability (Bond et al., 1998; Kroslak et al., 2007), and has also shown association with reduced OPRM1 mRNA expression (Zhang et al., 2005).
