BDNF and major depressive disorder: Baseline DNAm levels of BDNF in MDD patients in both blood and leukocytes was positively correlated with remission of depressive symptomatology after antidepressant treatment in 4 of 6 independent studies (Tadić et al., 2014; Lieb et al., 2018; Wang et al., 2018b; Hsieh et al., 2019).