BDNF and major depressive disorder: Through analyzing the expression levels of BDNF mRNA, acH3K9 and HDAC2, and the DNA methylation level of BDNF promoter I at relative CpG sites in hippocampus, we identified that DNA methylation and histone modifications of BDNF in the hippocampus were involved in the pathological process of depression (Figures 4H,I).