In this study, we sought to further explore the value of NfL and α-syn seeding activity in biofluids, either alone or in combination, as diagnostic and prognostic biomarkers in a large series of patients presenting with parkinsonism and related syndromes, including PD, MSA, PSP, CBS, dementia with Lewy bodies (DLB), PD dementia (PDD), isolated autonomic failure (iAF) and isolated REM sleep behavior disorder (iRBD). This evidence concerns the gene NEFL and supranuclear palsy, progressive, 1.