Three meta-analyses and a large cohort study [8]–[11] demonstrated that serum levels of BDNF were significantly lower in patients with MDD, although at the time, the commercially available human BDNF ELISA kits were unable to distinguish between proBDNF and mature BDNF because of the limited specificity of the BDNF antibody [18]. Here, BDNF is linked to major depressive disorder.