DRD4 and attention deficit-hyperactivity disorder: These studies examined the DNA variants of candidate genes and found evidence for a small number of potential genes that have been consistently associated with ADHD across studies, such as the D4 dopamine receptor gene (DRD4), the dopamine transporter gene (DAT1), and the serotonin transporter gene (5HTTLPR) (71, 72, 74, 75).