APOE and Alzheimer disease: Similarly to PD, gene therapy ongoing clinical trials in the AD field can also be categorized on the basis of the selected target: (i) neurotrophic factors from the GDNF family, brain-derived neurotrophic factor (BDNF), and beta-nerve growth factor (NGF), (ii) neuromodulators such as GAD, and (iii) specific mutations, particularly in apolipoprotein E (APOE).