Maximum variance explained in the testing sample was reached when the top four SNPs from the training sample association tests (along with their training-sample-derived regression weights) were aggregated into a genetic risk score. These top four SNPs (rs1611131, rs5326, rs9817063, and rs1079597) are located in four different genes (dopamine beta-hydroxylase, DBH; dopamine receptor D1, DRD1; dopamine receptor D3, DRD3; and dopamine receptor D2, DRD2; respectively). It should be noted that the negative regression weight on the DBH SNP shown in Table 2 was treated as such (when it was used to weight the DBH SNP for inclusion in the risk score), meaning that a greater number of the coded (in this case, protective) alleles would result in a lower risk score. This four-SNP score explained 2.76% of the sample variance in number of cocaine dependence symptoms in the training set from which the SNP weights were derived (p < 2.5 × 10−6). This same genetic risk score explained 0.55% (p = 0.037) of the variance in cocaine dependence symptoms in the independent testing sample. This suggests that the effects of
