APOE and Alzheimer disease: Gene delivery of nerve growth factor (NGF) [123], brain-derived neurotrophic factor (BDNF) [124], neprilysin [125], APOE [126], ECE [127], and cathepsin B [128] has been studied extensively in several AD animal models with promising results, and the first clinical trial using ex vivo gene delivery has been completed, with result indicating amelioration of AD pathogenesis [129].