The DRD4 48-bp VNTR polymorphism is in exon 3 of the gene coding for the dopamine receptor D4. The VNTR polymorphism varies between 2 and 11 repeats of a similar 48-bp coding region sequence, with a trimodal distribution of 2, 4 and 7 repeat alleles (2R, 4R and 7R) in most, but not all, populations [61]. Although the functional significance of the DRD4 VNTR polymorphism has not been definitively characterized, long alleles (typically 7R as opposed to 4R) have been generally found to be functionally less reactive in in-vitro expression experiments [62]–[66], with some heterogeneity [67]–[71]. Additionally, in vivo human pharmacological studies are also generally consistent with the notion that 7R alleles are associated with less responsive D4 receptors than 4R alleles [72]–[76].
