NEFL and Parkinson disease: We tested the hypotheses that NfL would (1) show a positive correlation between serum and CSF values, (2) be higher in PD participants than in healthy controls (HCs), (3) be even greater in participants with other cognate or neurodegenerative disorders (ONDs), (4) increase over time with disease progression, (5) be higher in prodromal and asymptomatic mutation carriers than in controls, and (6) correlate with clinical measures and/or imaging indices of progression in a single-center cohort (DeNoPa) and a large multicenter cohort (PPMI).