SLC6A3 and attention deficit-hyperactivity disorder: In this respect, motor system maturation must also be kept in mind, as, in children, the 6R–10R-allele has been described to be a risk factor for ADHD, while in adults the 6R–9R allele was associated with ADHD [62], [63], pointing towards a differential decay of dopamine transporter expression with increasing age [20].