Although additive genetic influences are important in the etiology of SUD, to date, they have not accounted for a substantial proportion of the total variance in SUD. Further, addiction is thought to result from changes in brain structure and function (Zhou, Enoch, & Goldman, 2014). As summarized in twin/family study results, additive genetic influences, alone, are not expected to contribute to SUD. Therefore, other genetically-mediated mechanisms may influence the regulation of gene expression in the etiology of SUD (Hyman, 2005; Impey, 2007; Jones & Bonci, 2005; Kalivas, 2005; Kalivas & O’Brien, 2008a; Kauer & Malenka, 2007; Koob & Le Moal, 2005; Lu, Koya, Zhai, Hope, & Shaham, 2006; McClung et al., 2004; Moghaddam & Homayoun, 2008; Mulholland & Chandler, 2007).
