A multitude of preclinical studies with GDNF protein in an array of rat, mouse and monkey models of PD have demonstrated potent effects of this factor in protecting DA neurons from neurotoxin-induced cell death as well as stimulating regrowth of nigral neurons in primate models of PD (Kozlowski et al., 2000; Eberling et al., 2009; Kells et al., 2010; Lindgren et al., 2012). This evidence concerns the gene GDNF and Parkinson disease.