Recently, more polymorphism in dopamine transporter genes have been reported in patients with ADHD, such as AGO1 rs595961 (63), SNAP-25 MnlI (64) and dopamine receptor D4 (DRD4) (9, 66), suggesting the role of gene variants in the pathogenesis of ADHD. Here, DRD4 is linked to attention deficit-hyperactivity disorder.