SLC6A3 and SLC6A3-related dopamine transporter deficiency syndrome: One additional neurochemical feature common for DAT-KO mice and DTDS patients is the elevated ratio of the extracellular DA metabolite HVA to a major serotonin metabolite, 5-hydroxyindolacetic acid (5-HIAA)19, as it is demonstrated in KO control mice in this study.