Changes in the BDNF expression and DNA methylation level are often seen in some of the early adversity related mental diseases (e.g., depression, schizophrenia, bipolar disorder, and autism), so that BDNF DNA methylation in the blood can be used as a novel biomarker for early detection of many mental diseases (Kundakovic et al., 2015). Here, BDNF is linked to major depressive disorder.