GABRA2 and CHRM2 previously observed with AD came only from individuals with comorbid DD, with no evidence of association among individuals with AD but no DD (Agrawal et al., 2006; Dick et al., 2007a). We conclude that TTC12 exon 3 is a risk region for AD+DD, with evidence of association with AD-all due only to the contribution by the comorbid AD+DD part of the sample. This suggests that the analysis of samples with drug dependence in the absence of alcohol dependence is warranted, to ascertain whether the locus predisposes to drug dependence specifically or, as suggested here, to the comorbid phenotype.
