SLC6A4 and major depressive disorder: An insertion/deletion (I/D) polymorphism in the gene encoding the serotonin transporter, also known as solute carrier family 6 member 4 (SLC6A4), located in the serotonin-transporter-linked polymorphic region (5-HTTLPR), has been reported to moderate the influence of stressful life events on depression (Caspi et al. 2003).