The dopamine D4 receptor is encoded by the DRD4 gene located on chromosome 11p15.5 [66]. The exon III region of the DRD4 gene contains a variable number of tandem repeats (VNTR) polymorphism, which are nucleotide sequences that repeat themselves a certain number of times [64, 68]. The function of long-repeat allelic variants (> 4 repeats) and short-repeat alleles has been identified in disorders such as ADHD and Parkinson’s disease. Presence of VNTR SNPs in ADHD has been shown to influence brain activation patterns of temporal processing and overall executive functioning [69]. In the case of substance use, the exon III VNTR polymorphism has been studied in association with addictive disorders, as hypothesized by Chien et al. [68]. After studying a sample of heroin-addicted Chinese men, Chien et al. reported a higher frequency of the long-repeat alleles in cases compared to controls, providing evidence for an association with heroin dependence [68]. A study of DRD4 exon III VNTR polymorphisms in Israeli subjects found that the 4 and 7 repeats of exon III account for 90% of the alleles in an Israeli
