This does not mean that such genes account for the majority of the genetic load associated with addiction; indeed, this is the real implication of GWAS. Even prior to the publication of the Liu et al (2006) study, the realization that the types of genes being identified in GWAS for drug dependence involved classes of genes likely to be involved in synaptic plasticity led to the suggestion that addiction is fundamentally a problem of altered mnemonic processes (G. R. Uhl, 2004). This finding is particularly interesting in the context of the emphasis on addiction phenomena such as craving and habit that are primarily conditioned responses (for review see (Robbins, Ersche, & Everitt, 2008)) and the evidence for synaptic changes after exposure to a variety of classes of addictive drugs (for review see (Badiani & Robinson, 2004)). The genes that have been identified in GWAS for drug dependence may be involved either in adjusting brain “wiring” prior to drug experiences or subsequent to drug experiences, or both. Based on this analysis, and consistent with the high comorbidity of drug dependence with
