Circulating human lymphocytes express functional serotonin transporter (SERT; encoded by SLC6A4)5, 6, 7, 8 that exhibits a pharmacology similar to those of the brain and platelet SERTs9, 10 and may therefore serve as an important means to understand the mode of action of antidepressant drugs and treatment resistance in MDD. This evidence concerns the gene SLC6A4 and major depressive disorder.