Individuals with p-tau217 in the top quartile were 8 times more likely to phenoconvert to PD-CI or DLB compared to the bottom quartile (hazard ratio = 8.30; 95% confidence interval, 2.49-27.65).<h4>Interpretation</h4>Serum p-tau217, but not NfL, is a useful biomarker of cognitive impairment in iRBD that could be integrated into a multimodal prognostic indicator when stratifying risk of phenoconversion. The gene discussed is NEFL; the disease is Lewy body dementia.