In addition, BDNF levels in blood (e.g., serum and plasma) have been extensively examined as a possible biomarker reflecting mental conditions, such as depression (Karege et al. 2002; Sen et al. 2008; Molendijk et al. 2014), schizophrenia (Fernandes et al. 2015), and Alzheimer’s disease (Yasutake et al. 2006), because measuring BDNF levels in the brains of living humans is difficult. This evidence concerns the gene BDNF and major depressive disorder.