SLC6A4 and major depressive disorder: They concluded, “there is no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity.” The review included investigations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in “body fluids”, 5HT1A receptor and serotonin transporter protein (SERT) availability in imaging and post-mortem studies, investigations of SERT gene polymorphisms, interactions between SERT and stress in depression, and effects of tryptophan depletion on mood.