SLC6A4 and major depressive disorder: It is noteworthy to mention that a significant association of six genes with MDD susceptibility has been revealed by meta-analyses of genetic studies, and the identified genes include APOE-SLC6A4-apolipoprotein E, SLC6A3- dopamine transporter and SLC6A4-serotonin transporter, DRD4-dopamine receptor D4, MTHFR-methylenetetrahydrofolate reductase, and GNB3-guanine nucleotide-binding protein [12].