One of the polymorphic sites in the SLC6A4 is an insertion/deletion in the 5′-flanking promoter region (serotonin transporter-linked polymorphic region, 5-HTTLPR), which is associated with MDD-related phenotypes, including vulnerability to MDD9 and response to antidepressant treatment.10 This evidence concerns the gene SLC6A4 and major depressive disorder.