Rodent and nonhuman primate models of PD, which mostly include neurotoxins MPTP and 6-OHDA, have shown that ectopic GDNF alleviates motor symptoms, enhances dopamine levels in the brain, and protects dopamine neurons from degeneration (Hoffer et al., 1994; Kearns and Gash, 1995; Gash et al., 1996; Kearns et al., 1997; Oiwa et al., 2002; Kordower et al., 2006). Here, GDNF is linked to Parkinson disease.