Our clinical subjects form an extreme sample in terms of drug abuse in line with the ascertainment criteria (having at least one drug dependence symptom, excluding ND criteria). Over 70% of the CADD clinical subjects have at least one substance use disorder other than ND. The GADD sample is similar in that 67% of the sample has one or more substance use disorders. We did not control for the use of other substances in our study, and given the extreme nature of this sample, this may limit the generalizability of these results. Additionally, in this analysis we have included subjects who have not tried tobacco products. While many groups limit their samples to subjects meeting a certain use threshold (e.g., Saccone et al. 2009; Saccone et al. 2010) others have found associations with the CHRNA4 gene among samples including individuals who have never smoked (Han et al. 2011). This is an important limitation to this study and it is possible that this association is driven by tobacco use rather than dependence specifically, but unfortunately we do not have enough subjects to test this hypothesis.
