The serum GDNF concentration of the PD-N group (532.13 ± 138.30 pg/mL) was significantly higher than that of the PD-MCI group (439.87 ± 139.59 pg/mL, P < 0.01) and the PD-D group (424.73 ± 101.96 pg/mL, P < 0.01). This evidence concerns the gene GDNF and Parkinson disease.