Our result, that GDNF can provide its neuroprotective and neuroregenerative function on midbrain dopaminergic neurons only in the presence of the receptor tyrosine kinase Ret, has practical implications for therapeutic strategies to treat PD with GDNF and neurturin.3, 7, 11, 12, 13 It suggests that the main goal of clinical trials using GDNF and related substances should be to activate the Ret receptor. This evidence concerns the gene NTRK1 and Parkinson disease.