GDNF and Parkinson disease: Currently, bone marrow-derived mesenchymal stem cells (BM-MSCs) are one type of stem cell that is being used to treat HD, AD and PD by genetically overexpressing proteins that are downregulated in each disease, such as brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and glial-derived neurotrophic factor (GDNF), respectively [184,185,186].