The APOE gene has three alleles: APOE2, APOE3, and APOE4. Compared to the most common APOE3 form, APOE2 is neuroprotective2, while APOE4 increases AD risk; APOE4 homozygotes are 15-fold more susceptible to AD than APOE3 homozygotes3–5, and disease begins several years earlier in APOE4 carriers than those with APOE3 or APOE23. This evidence concerns the gene APOE and Alzheimer disease.