APOE and Alzheimer disease: The apolipoprotein gene (APOE) on chromosome 19, also implicated in AD, harbors common variants: ε2, considered protective for AD (~7% of the general population); ε3, the most common allele (~79% frequency), neutral regarding AD risk; and ε4 (~14% frequency), thought to harbor the greatest risk for AD, particularly in carriers of two ε4 alleles.APOE variants modulate the age of onset of AD in DS26.