Brain-derived neurotrophic factor (BDNF) has key roles in neuronal differentiation and growth, synapse formation and plasticity, and higher cognitive functions.1 The neurotrophin hypothesis of depression states that reduced levels of brain BDNF are associated with depression.2, 3 In the early 21st century, low serum BDNF concentrations in depressive patients4 and increased BDNF concentrations during the course of antidepressant treatment5 were reported. This evidence concerns the gene BDNF and major depressive disorder.