BDNF and depressive disorder: 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.