SLC6A4 and substance abuse: Alterations of serotonergic neurotransmission have been critically implicated in the etiology of several psychopathologies including mood and anxiety disorders and substance abuse.1, 2, 3, 4 The serotonin transporter (SERT, SLC6A4) is the major regulator of serotonergic tone in the brain through the control of the reuptake of released serotonin from the synaptic cleft into the presynaptic terminal.