GDNF and Parkinson disease: Additional follow-up studies investigated and established the neuroprotective effects of GDNF protein or GDNF gene vectors on DA neurons in rodent and monkey PD models (Gash et al., 1995, 1996; Choi-Lundberg et al., 1997; Kordower et al., 2000; Nakajima et al., 2001; Grondin et al., 2002; Maswood et al., 2002; Ai et al., 2003; Kirik et al., 2004; Rangasamy et al., 2010).