SLC6A4 and attention deficit-hyperactivity disorder: In the 5-HT pathway, the 5-HT synthesis enzymes TPH1 and TPH2 (Walitza et al., 2005; Li et al., 2007) and the 5-HT transporter gene (SERT/SLC6A4; Gizer et al., 2009) have all been linked to ADHD formation.