GDNF and Parkinson disease: There are three broad categories of neurotrophic factors implicated in PD; neurotrophins such as brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF)-family ligands, and the cerebral dopamine neurotrophic factor/mesencephalic astrocyte-derived neurotrophic factor (CDNF/MANF) family [17].