Lack of efficacy in the Phase II CERE-120 study [94], despite the predicted results from the mentioned preclinical investigations for both GDNF and NTRN, raise questions regarding: the appropriateness of the animal models used [226]; possible abnormal retrograde transport to SN due to degeneration of the nigrostriatal pathway in PD [94]; and/or, a lack of significant target coverage within the putamen [97]. Here, GDNF is linked to Parkinson disease.