BDNF and major depressive disorder: Epigenetic changes have been well-established in BDNF, COMT, FKBP5, NR3C1, SLC6A4, and DRD2, genes associated with psychiatric disorders, including schizophrenia, major depressive disorder (MDD), bipolar disorder (BP), post-traumatic stress disorder (PTSD), and autism spectrum disorder (ASD).