Based on the etiology of ADHD, we chose candidate genes that encode functionally relevant proteins involved in noradrenergic (SLC6A2, ADRA2A), dopaminergic (SLC6A3, DRD2, DRD4, COMT, DDC), and serotonergic (SLC6A4, HTR2A, HTR2C) neurotransmission. Here, SLC6A4 is linked to attention deficit-hyperactivity disorder.