Accumulating evidence suggests that the brain-derived neurotrophic factor (BDNF) and its specific receptor, tropomyosin-related kinase (TrkB) signaling, have an important role in the pathophysiology of depression and drug addiction.11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 This would make BDNF–TrkB signaling a potential therapeutic target for depression and drug addiction. This evidence concerns the gene NTRK2 and major depressive disorder.