Zou et al. performed a network meta-analysis on different psychiatric disorders, and found that peripheral BDNF concentration in patients with MDD, bipolar disorder, obsessive–compulsive disorder, panic disorder, or schizophrenia is lower compared with that in controls, although patients with PTSD showed increased BDNF levels, suggesting BDNF is a valuable biomarker for the diagnosis of mental illnesses [61]. This evidence concerns the gene BDNF and major depressive disorder.