It has been suggested that dysfunctions of dopaminergic systems in the brain contribute to the pathogenetic basis of ADHD, because the amount of dopamine is reduced in the striatum and prefrontal cortex of ADHD patients [3], and methylphenidate (MPH), a dopamine transporter inhibitor, reverses the down-regulation of dopamine in these areas and improves abnormal behavior in ADHD patients [4]. This evidence concerns the gene SLC6A3 and attention deficit-hyperactivity disorder.