In the case of the 5-HT system, the 5-HT synthesis enzymes TPH1 and TPH2 [319,320,321] and transporter genes serotonin transporter (SerT)/solute carrier family 6 member 4 (slc6a4) which encodes an integral membrane protein that transports the neurotransmitter serotonin from synaptic spaces into presynaptic neurons, have also been demonstrated to impact ADHD phenotypes [322]. The gene discussed is SLC6A4; the disease is attention deficit-hyperactivity disorder.