A study employing the serotonin transporter (SERT) mutant rats has shown that deletion of the SERT gene is associated with an increased level of proinflammatory cytokines and an increased number of activated microglia suggesting that the serotonin transporter dysfunction plays an important role in the pathogenesis of depressive disorders via an increased glial response to inflammatory agents [92, 93]. Here, SLC6A4 is linked to depressive disorder.