In recent years considerable studies on different candidate genes for ADHD have been identified such as the catechol-O-methyltransferase (COMT) gene (Tomlinson et al., 2015), the dopamine active transporter 1 gene (DAT1, also known as SLC6A3) and the dopamine receptor D4 (DRD4) gene (Faraone et al., 2014). Here, DRD4 is linked to attention deficit-hyperactivity disorder.