In recent years, neurotrophic factors including brain-derived neurotrophic factor (BDNF), vascular endothelial growth factor (VEGF), nerve growth factor (NGF), glial cell-line derived neurotrophic factor (GDNF), and insulin-like growth factor (IGF-1) have been widely explored in depression-related studies (2–5). Here, NGF is linked to major depressive disorder.