SLC6A4 and major depressive disorder: Recently, it has been found that the expression levels of miR-135 decrease in the serotonergic raphe nucleus of individuals having major depression disorder (MDD) and that miR-135 targets both the gene encoding the serotonin transporter (SERT) and the gene encoding 5-hydroxytryptamine receptor 1A (HTR1A) in vitro in mouse tissue [77].