PSEN2 and Alzheimer disease: Among the ε2–ε4 polymorphic alleles, the ε4 allele of the apolipoprotein E (ApoE4) gene is the strongest genetic risk factor for late onset Alzheimer’s disease (LOAD)1,2, whereas inherited genetic mutations in amyloid precursor protein, presenilin 1, or presenilin 2 genes cause rarer early onset familial Alzheimer’s disease (AD)3.