GDNF and Parkinson disease: There exist possible explanations, which include a reduction in Nuclear-receptor-related 1 (Nurr1) expression [65], the use of GDNFs in late-stage Parkinson’s, where it would not be as effective [66], inaccurate representations of PD in preclinical animal models [67], and poorly designed catheter systems in the intracranial delivery of GDNF [68].