NEFL and Parkinson disease: The serum NFL concentrations were significantly higher in patients with PD (16.6 ± 3.5 pg/ml) than in patients with ET (12.2 ± 2.4 pg/ml) and in healthy controls (11.8 ± 2.4 pg/ml) (both p < 0.01; effect sizes = 1.47 and 1.60, respectively; Table 1 and Fig. 1A).