Recent meta-analyses of candidate gene association studies have found consistent evidence of significant associations between ADHD and polymorphisms in several candidate genes that are almost exclusively involved in the regulation of dopaminergic and serotonergic transmission (including the dopamine transporter (DAT1) gene, the dopamine D4 receptor (DRD4) gene, the dopamine D5 receptor (DRD5) gene, the serotonin transporter (5-HTT) gene, the 5-hydroxytryptamine receptor 1B (HTR1B) gene, and synaptosomal-associated protein 25 (SNAP25) [440,441]. This evidence concerns the gene DRD4 and attention deficit-hyperactivity disorder.