BDNF and major depressive disorder: Across studies, altered methylation of <i>NR3C1</i>, <i>FKBP5</i>, and <i>BDNF</i>, reduced hippocampal histone acetylation, and shifts in microRNA profiles (miR-16, miR-124, miR-132, miR-135a, miR-34c) were repeatedly associated with HPA axis dysregulation, limbic system remodeling, and phenotypes relevant to PTSD and depression.