SLC6A4 and major depressive disorder: Hypermethylation of BDNF, CRMP2, NR3C1, and SLC6A4 is associated with Depression and MDD (Xiang et al., 2021, Li et al., 2021, Borçoi et al., 2021, Li et al., 2019, Schiele et al., 2021, Sanwald et al., 2021, de Assis Pinheiro et al., 2021).