Family-based and case–control studies of candidate genes have replicated significant linkage and/or association between ADHD and variants in dopamine receptors (DRD4, DRD5), dopamine transporter (SLC6A3), serotonin transporter (SLC6A4), serotonin receptor (HTR1B), and proteins involved in synaptic transmission (SNAP25, LPHN3)6,8–16, all of them contributing to small- to medium-sized effects. The gene discussed is SLC6A3; the disease is attention deficit-hyperactivity disorder.