APOE and Alzheimer disease: Although the APOE2, APOE3 and APOE4 alleles are normally present at a relative frequency of about 8%, 78% and 14%, respectively, the APOE4 allele is present at a relative frequency of about 37% in AD patients5, with individuals who possess one or two APOE4 alleles having an odds ratio for AD of about 3 or 12, respectively5,6.