And there was no notable enhancement in the motor function of participants in placebo-controlled studies, which may be due to a lack of improvement attributed to the ineffectiveness of GDNF in enhancing the condition of damaged dopamine neurons (Barker et al., 2020), possibly suggesting that GDNF acts mainly during the initial phases of PD or primarily makes a preventative contribution. This evidence concerns the gene GDNF and Parkinson disease.