Many studies have shown that the ɛ4 allele is associated with dementia in PD,20,24–26 and potentially separately from the risk of Alzheimer’s disease (AD).27 One possible mechanism is that APOE is associated with amyloid-β pathology, as comorbid AD pathology is common in PD patients with dementia (PDD) at postmortem.28 Alternatively, APOE may drive cognitive decline independently of amyloid/AD pathology. This evidence concerns the gene APOE and early-onset autosomal dominant Alzheimer disease.