While GDNF has achieved significant results in rodents (Beck et al., 1995; Kozlowski et al., 2000; Rosenblad et al., 2000) and nonhuman primates (Gash et al., 1995, 1996, 2005; Zhang et al., 1997; Ai et al., 2003), it has not given therapeutic benefits in PD patients in clinical trials. This evidence concerns the gene GDNF and Parkinson disease.