Brain-derived neurotrophic factor plays a crucial role in neuronal growth, development, and synaptic plasticity and acts as a neuromodulator.25 Pro-BDNF, a precursor of BDNF, has the opposite function and promotes depression and anxiety.26 Brain BDNF can cross the blood-brain barrier into the circulation, with brain BDNF level positively correlating with blood BDNF level.27 Therefore, the level of BDNF in peripheral blood roughly mirrors its level in the central nervous system. This evidence concerns the gene BDNF and major depressive disorder.