NGF and Alzheimer disease: Moreover, studies that used stem cells that over-express different neurotrophic factors, such as BDNF, acetylcholine (Ach), and nerve growth factor (NGF), found that the delivery of these genetically modified stem cells to animal models of AD is safe and effective to restore learning and memory functions, thus suggesting that stem cell-based gene therapy may be a promising treatment for AD29, 53, 57, 58, 77.