GDNF and Parkinson disease: Probably the best-studied NTFs in the context of PD are glial cell line-derived neurotrophic factor (GDNF) and another GDNF-family ligand neurturin (NRTN), brain-derived neurotrophic factor (BDNF), CDNF and mesencephalic astrocyte-derived neurotrophic factor (MANF) (Fig. 3).