Our approach of selecting only SNPs located in genes known to be directly involved in a neurotransmitter system of interest aimed to reduce the extent to which “noise” SNPs could be included in our genetic risk score. Such an approach also substantially limited the amount of variance in cocaine dependence that we would expect to explain. Although other non-dopamine systems, such as those involved in cocaine metabolism, likely contribute to individual differences in liability to cocaine addiction, the current total sample size of 1591 individuals who reported having ever used cocaine required that a trade-off be made in terms of specificity of our target system (to reduce the chance of false-positives as a result of multiple testing) versus generality at the level of either the inclusion of multiple systems or even taking a genome-wide scoring approach. As a result, we were unable to compare the results of the current limited-scope investigation to the results of a potential multiple system or genome-wide scoring approach with regard to genetic influences on cocaine dependence. (To our knowledge there is currently no published GWAS
