APOE and Parkinson disease: Genetic interactions refer to a combination of two or more genetic variants whose phenotypic contribution is amplified by their co-occurrence.21 To determine if genetic-level interactions between SNCA and APOE exist that can explain the protein-level association as described, we investigated the genotypic interaction of APOE and SNCA in three disease cohorts of PD, DLB and iRBD patients and controls, with a total of 8855 patients and 11 067 controls.