Defective BDNF expression and/or function is thought to be associated with depression, schizophrenia, Alzheimer's disease (AD), Parkinson's disease (PD), and ALS (Hyman et al., 1991; Phillips et al., 1991; Holsinger et al., 2000; Castrén et al., 2007; Favalli et al., 2012; Libman-Sokołowska et al., 2015). Here, BDNF is linked to major depressive disorder.