Thus, translational research on NTFs' neuroprotective and neurorestorative effects on DA neurons in PD monkey models and clinical trials have been focused on GDNF (Gash et al., 1995, 1996; Kordower et al., 2000; Kozlowski et al., 2000; Connor et al., 2001; Grondin et al., 2002; Maswood et al., 2002; Ai et al., 2003; Gill et al., 2003; Cohen et al., 2011; Kordower and Bjorklund, 2013; Olanow et al., 2015; Bartus and Johnson, 2017a,b; Kirik et al., 2017). Here, GDNF is linked to Parkinson disease.