Recently, we were able to show that – across ADHD patients and controls – there was no main effect of two common gene variations (i.e. catecholamine-O-methyl transferase [COMT] val158met polymorphism; dopamine receptor D4, DRD4 variable number of tandem repeats [VNTR]) on behavioral performance or the NGA, but an interaction between the gene variants on the investigated parameters [25]. This evidence concerns the gene DRD4 and attention deficit-hyperactivity disorder.