NEFL and Parkinson disease: Age and CSF NfL levels were significantly different between three groups, meanwhile age positively correlated with CSF NfL in each group (HC: β = 2.2983, p < 0.001; PD-NC: β = 2.3492, p < 0.001; PD-MCI: β = 3.5410, p < 0.001; Supplementary Figure 2) which was also consistent with reported study (Lerche et al., 2020; Fagan et al., 2021).