Although all of the variants genotyped in this study have some biological function ascribed to them, it should be noted that three of the four SNPs are rare in the population and the rare variant carriers in our drug addicted populations were all heterozygotes. The functional consequences of carrying a single mutant allele may be different to those observed when homozygous mutants are expressed in cell lines. If the ‘wild-type’ allele is dominant, then heterozygous [13, 14, 23, 26, 33] individuals would have no distinct phenotype. This effect may account for the lack of association observed between these alleles and drug addiction; however, it is also plausible that they are simply not relevant for addiction risk.
