Smaller global and regional brain volumes and subcortical structures together with multiple gene polymorphisms are reported as the risk factors for ADHD (Table 1), and one such example of gene polymorphism is Dopamine transporter solute carrier family 6 member 3 (SLC6A3); previously known as DAT1 gene which encodes the transmembrane proteins involved in reuptake of dopamine from the synapse (Fig. 8). Here, SLC6A3 is linked to attention deficit-hyperactivity disorder.