GDNF and Parkinson disease: Since its discovery in 1993 [1], glial cell line-derived neurotrophic factor (GDNF) has shown consistent potential as a neuroprotective and neurorestorative therapy in a succession of in vitro[1]–[3]; [4]–[6] and in vivo studies in the 6-hydroxydopamine (6-OHDA) rat model of PD [4]–[6] and in aged and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned non-human primates (NHPs) [7]–[10].