Serum samples from a REM sleep behaviour cohort stratified by risk of progression to overt Parkinson’s disease (high risk versus low risk) and from early-stage Parkinson’s disease patients and controls (Table 1 and Fig. 1A) were applied to a custom MSD assay for antibodies to disease relevant proteins, including alpha-synuclein, alpha-synuclein fibrils, S129D peptide, Y39 peptide and tau. This evidence concerns the gene SNCA and Parkinson disease.