Stem cell-based therapy may be more effective against AD than conventional drug therapies because implanted stem cells can improve the brain microenvironment by supplying growth-promoting and growth-permissive factors for synaptogenesis and neurite repair, reduce oxidative stress by enhancing local antioxidant capacity, and stimulate the sustained production of neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) [27]. Here, NGF is linked to Alzheimer disease.