Researchers unveiled an endpoint dilution seed amplification assay (SAA) that quantifies cerebrospinal fluid α-synuclein seeds with improved precision, delivering a more granular biomarker for Parkinson’s disease and synucleinopathies. The method refines sensitivity and reproducibility across dilution series, enabling estimation of seed concentration rather than binary detection. Authors report validation across control and disease cohorts and discuss analytic thresholds for clinical translation. Seed amplification assays detect misfolded protein aggregates that nucleate pathology; this quantitative advance could enable staging, pharmacodynamic readouts, and patient selection for α-synuclein–targeted therapies.