In addition to the previously indicated dopaminergic polymorphisms, namely the 40 bp VNTR (variable number of tandem repeats) of the DAT1 gene, and the Val158Met polymorphism of the COMT gene, the most commonly investigated DRD2 and DRD4 polymorphisms were genotyped. The DRD2 TaqIA polymorphism was identified during the chromosomal localization of the gene. However, this single nucleotide polymorphism (SNP) is in the 3' untranslated region, 10 kilobase downstream from the DRD2 gene, actually in the neighboring ANKK1 gene [18]. Nevertheless, four independent workgroups showed reduced D2 receptor density of the minor A1-allele carriers [19-22], and only one reported a negative finding [23]. Therefore, this polymorphism might be a good marker for DRD2 density in the brain. The DRD2 gene has been associated with alcoholism and other addictive disorders [24], and is hypothesized to be a reinforcement or reward gene. Recently, two studies showed an association between the DRD2/ANKK1 A1-allele and impulsive behavior among healthy young adults [25,26]. To further assess the possible involvement of the DRD2 gene, two additional SNPs (TaqIB from intron 1 and TaqID from intron 2) were selected for genotyping.
