For example, emerging evidence indicates that hypermethylation of the brain-derived neurotrophic factor (BDNF) promoter is observed not only in Rett syndrome and Alzheimer's disease but also in Parkinson's disease, major depressive disorder, and schizophrenia, where it has been linked to impaired synaptic function, reduced neurotrophic support, and heightened neuronal vulnerability [46-50]. Here, BDNF is linked to major depressive disorder.