SLC6A4 and attention deficit-hyperactivity disorder: Our results add to extensive literature information about polymorphic variants in genes whose implication in ADHD is widely known through the pathophysiology as SLC6A2, SLC6A3, SLC6A4 and LPHN3. In some cases the polymorphism associated has a known functional implication, like rs28386840, a functional promoter variant of SLC6A2 gene.