APOE and Alzheimer disease: The fact that APOE ε4 is the most predictive variant for amnestic AD but does not appear to be associated with risk for atypical AD syndromes such as PCA and lvPPA [59] likely contributes to the decreased specificity of the genetic risk assessment; namely, carrying an ε4 allele is associated with being affected in amnestic AD but is also associated with not being affected by PCA or lvPPA.