While previous studies have shown the value of plasma NfL as a cognitive severity biomarker in PD and DLB patients,13, 15, 16, 17, 18 the evidence in iRBD is limited, with only one study showing that a higher baseline level of plasma NfL in iRBD patients is associated with a higher risk for cognitive progression.22 This evidence concerns the gene NEFL and Parkinson disease.