The most numerous studies suggest the effects of the dopamine pathway genes: The dopamine D4 receptor gene (DRD4), the dopamine D5 receptor gene (DRD5), and the dopamine transporter gene (DAT1) [33], but more recent publications also report the potential role of the serotonin receptor family (5-HTR) [34,35] and the serotonin transporter (5HTT) [35] in the pathogenesis of ADHD. Here, DRD4 is linked to attention deficit-hyperactivity disorder.