APOE and early-onset autosomal dominant Alzheimer disease: The apolipoprotein E4 (APOE4) genotype is the strongest prevalent genetic determinant of Alzheimer’s disease (AD) risk with heterozygotes (APOE3/E4) and homozygotes (APOE4/E4) carriers at 3–4- and 12–15-fold increased risk, respectively, relative to the “wild-type” APOE3/E3 genotype [1,2].