MAOA and attention deficit-hyperactivity disorder: The genes correlating with HI included catechol-O-methyltransferase (COMT), dopamine receptor D2 (DRD2), monoamine oxidase A (MAOA), and solute carrier family 6 (neurotransmitter transporter, serotonin), member 4 (SLC6A4) – all having previously been associated with ADHD (Table 2).