SLC6A2 and attention deficit-hyperactivity disorder: 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.