The hypothesized role of a dysfunctional mesocortical dopaminergic pathway in the development of symptoms of ADHD has encouraged the investigation of candidate genes involved in this pathway including SLC6A3 [9], DRD4 [10] and, more recently, the catechol-O-methyltransferase (COMT) gene [24]. This evidence concerns the gene SLC6A3 and attention deficit-hyperactivity disorder.