Initially, it was expected the first significant hits to be revealed in these GWAS would be dopaminergic and noradrenergic genes, which were previously implicated in ADHD by meta-analysis of candidate gene studies, such as those encoding the dopamine receptors (e.g. DRD2, DRD4), the dopamine transporter (DAT1) and the enzymes monoamine oxidase (MAOA), catechol-O-methyltransferase (COMT) and dopamine beta-hydroxylase (DBH) [54, 55]. This evidence concerns the gene SLC6A3 and attention deficit-hyperactivity disorder.