GDNF and Parkinson disease: Most studied NTF in PD. GDNF was discovered by Lin et al. (1993). In 1994 the first in vivo study with GDNF demonstrated that intranigrally injected GDNF improved apomorphine-induced rotational behavior in 6-hydroxydopamine (6-OHDA)-injected rats (Hoffer et al., 1994). Six clinical trials were conducted with GDNF (Gill et al., 2003; Nutt et al., 2003; Slevin et al., 2005; Lang et al., 2006; Heiss et al., 2019; Whone et al., 2019). In these trials, GDNF was injected into the brain in the form of protein or in the form of gene therapy with viral vector.