There is a common three-allele APOE polymorphism: APOE*2, APOE*3, and APOE*4, resulting in six genotypes (2/2,2/3,2/4,3/3,3/4,4/4), and it has a profound effect on determining interindividual variation in plasma cholesterol level and in determining Alzheimer’s disease (AD) risk [2–5]. Here, APOE is linked to early-onset autosomal dominant Alzheimer disease.