In our previous study, we found that the serum tissue plasminogen activator (tPA)-brain-derived neurotrophic factor (BDNF) pathway was involved in the pathogenesis of MDD and the combination of serum tPA, BDNF, precursor-BDNF (proBDNF), tropomyosin-related kinase B (TrkB) and neurotrophin receptor p75 (p75NTR) in the pathway gave a better diagnostic power than any single protein21. Here, NTRK2 is linked to major depressive disorder.