DRD4 and attention deficit-hyperactivity disorder: Asians, including Koreans, are known to rarely exhibit the 7-repeat allele of DRD4 exon III 48-bp VNTR polymorphism [27,28], and the 4-repeat allele of the exon III VNTR polymorphism of the DRD4 gene was reported to be associated with response to methylphenidate treatment in Korean children with ADHD, suggesting that person with this allele would be more susceptible to external influences.