SLC6A4 and attention deficit-hyperactivity disorder: The exact genetic mechanism is yet to be elucidated; however, multiple genes have been postulated in the pathogenesis of ADHD in TS patients, which includes catechol-O-methyltransferase (COMT), dopamine receptor D2 (DRD2), monoamine oxidase A (MAOA), solute carrier family 6, member 4 (SLC6A4), myelin-associated oligodendrocyte basic protein (MOBP), dopamine receptor D1 (DRD1), and fatty acid desaturase 2 (FASD2) [27, 28].