Glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) GDNF (Figs. 1C, D and 3A) and NRTN have been shown to be efficient in protecting DA neurons in rodent and non-human primate (NHP) models of PD, but have only shown modest effects in Phase II clinical trials in PD patients [5, 13, 14]. This evidence concerns the gene GDNF and Parkinson disease.