SLC6A3 and attention deficit-hyperactivity disorder: The variable number of tandem repeat (VNTR) located on the 3′-untranslated region (3′-UTR) of the dopamine transporter gene (DAT1) and the VNTR in exon III of the dopamine D4 receptor gene (DRD4) are the two polymorphic systems most studied and most frequently replicated in relation to ADHD.