GDNF and Parkinson disease: Glial cell line–derived neurotrophic factor (GDNF) has been considered the most promising neurotrophic factor, showing positive effects in several animal models of PD (Hoffer et al. 1994; Kearns and Gash 1995; Tomac et al. 1995a; Gash et al. 1996; Zhang et al. 1997; Kirik et al. 2004), but not in the α-synuclein model of PD (Decressac et al. 2011).