DRD4 and attention deficit-hyperactivity disorder: Specifically, the seven-repeat allele in the D4 receptor (DRD4-7R), the 10-repeat variant in the dopamine transporter (DAT1) gene, and other polymorphisms in the D2 and D5 dopaminergic receptors, and in the gene coding for catechol-O-methyl-transferase have been associated with ADHD (Carrasco et al., 2006; Faraone and Khan, 2006; Swanson et al., 2007).