DRD4 and attention deficit-hyperactivity disorder: In contrast, a genome-wide association study identified candidate genes implicated in ADHD; these included SLC6A3 (dopamine transporter), DRD4 (dopamine receptor D4), DRD5 (dopamine receptor D5), CDH13, FOXP2, DUSP6, and LPHN3 (Gallo and Posner, 2016; Demontis et al., 2019; Grimm et al., 2020).