illicit drug dependence and when those individuals were excluded, evidence for association with AD disappeared (Agrawal et al., 2006). In contrast, excluding European American AD cases comorbid for drug (cocaine and/or opioid) dependence increased the strength of association between GABRA2 and AD (Covault et al., 2004). However, while Drgon et al. (2006) did not find association between six GABRA2 SNPs and AD in their European American case and control sample, they did observe a trend towards significance (p = 0.052) with polysubstance abuse among African Americans.
