BDNF and major depressive disorder: A major reason for the large number of publications reporting on BDNF levels in serum is the intriguing association with numerous conditions including depression (Karege et al., 2002; Sen et al., 2008) and neurodegenerative disorders such as Alzheimer’s disease (Laske et al., 2006; Lee et al., 2009; Forlenza et al., 2010), Parkinson’s disease (Scalzo et al., 2010; Costa et al., 2015; Wang et al., 2016), and Huntington’s disease (Ciammola et al., 2007; Zuccato et al., 2011).