Genetic factors are candidate predictors of cognitive decline and dementia in PD (PDD), although heterogeneity in the design of published studies has contributed to inconsistent findings.3 Among the strongest candidates are variants in the apolipoprotein E (APOE), β-glucocerebrosidase (GBA), microtubule-associated protein tau (MAPT), and α-synuclein (SNCA) loci. This evidence concerns the gene SNCA and Parkinson disease.