DRD4 and attention deficit-hyperactivity disorder: The choice of this refined ADHD phenotype of persistent medication responders echoes the central role of the DA dysregulation theory played in ADHD pathophysiology and etiology, i.e., the beneficial effects of stimulant treatment implicating DA dysregulation in ADHD and thus the involvement of dopamine system genes, including DRD4. Second, only boys are recruited in previous Leung et al.’s study [31] and this study.