GDNF and Parkinson disease: Glial cell line-derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), and mesencephalic astrocyte-derived neurotrophic factor (MANF) have shown neuroprotective and neurorestorative effects on lesioned dopaminergic neurons in vitro and in various animal models of PD [8–18].