amino acid substitution (Asn40Asp) in the extracellular portion of the μ opioid receptor. Although that polymorphism appears to be functional, there is controversy as to whether the change associated with the minor (Asp40) allele is a gain or loss of receptor function (Edenberg and Kranzler 2005). Further, multiple studies have shown association for the more common (Asn40) allele, and meta-analysis showed no overall association to substance dependence, including AD (Arias et al. 2006).
