BDNF and major depressive disorder: Several hypothesis-driven efforts have been made to identify trait and state biomarkers, and the search for peripheral biomarkers of depression has mainly been driven by the monoaminergic, the immune-inflammatory and the neuroendocrine hypothesis.3 Since the first paper reporting differences in serum brain-derived neurotrophic factor (BDNF) levels between depressed individuals and healthy controls,4 the hypothesis of a dysregulation in the neurotrophin system in depression has also been investigated.