The significantly increased levels of GDNF in the serum of patients with PSP-P and less increased in PSP-RS may suggest that in PSP, the levels of GDNF in the serum may initially rise significantly as a protective mechanism, which may partly come up with results of studies on GDNF in PD, where the factor inhibited the deterioration of certain clinical features. This evidence concerns the gene GDNF and Parkinson disease.