The APOE ε4 allele is the strongest known genetic risk factor for sporadic AD, and is a risk factor for DLB,11,12 CAA,13 and SVD.14 Despite these associations, it remains unknown if APOE ε4 modulates the relationship between WMH and cognition across the dementias, i.e., if APOE ε4 is an effect modifier in this association. This evidence concerns the gene APOE and Alzheimer disease.