Recognizing the rarity of this AD variant, we formed an international consortium comprising eleven centers, using clinical diagnostic criteria to define cases of PCA, with the principal aim of determining whether APOE ε4 and genetic risks from recent genome-wide association studies (GWAS) of AD and dementia with Lewy bodies (DLB, see below) are risk factors for the PCA variant of AD. The gene discussed is APOE; the disease is Alzheimer disease.