For example, brain-derived neurotrophic factor (BDNF) has drawn particular attention in the literature for its wide-ranging effects on the central nervous system, including significant associations with schizophrenia [5,6,7], depression [8] and anxiety [9,10], mood disorders [11,12], bipolar disorder [13,14], and cognition [15,16]. Here, BDNF is linked to depressive disorder.