SLC6A4 and major depressive disorder: In addition, genetic risk factors [12] such as genetic polymorphism of tumor necrosis factor-alpha (TNF-α) (-308G/A; rs1800629) [17,18,19,20,21], interleukin (IL)-1β (rs1143627) [22], IL-6 (174G/C; rs1800795) [23], 5HTTP/SLC6A4 (rs25531), APOE, DRD4, GNB3, HTR1A, MTHFR, and SLC6A3 have been significantly associated with MDD [24,25].