GDNF and Parkinson disease: Finally, PD is also an extremely heterogenous disorder: Progression rates vary widely, there is heterogeneity in the predominant symptom (e.g., tremor-dominant vs. gait/balance-dominant) that may not respond the same to GDNF, or may not be homogeneous enough for current progression markers (like UPDRS), to detect changes, Thus, as crucial as future biomarkers are in PD for earlier definitive diagnosis (Parnetti et al., 2019) and to track progression, they will be equally important to enable definitive clinical trials in early disease.