Genetic polymorphisms at the 5′ UTR promoter region of SLC6A3 have been proposed to be functional and known to alter transcriptional activity [12], and that these variants found in the promoter region may have a role in several psychiatric disorders, including attention deficit-hyperactivity disorder [37], bipolar disorder [38], and schizophrenia [39]. Here, SLC6A3 is linked to attention deficit-hyperactivity disorder.