APOE and Alzheimer disease: The protein products of three key genes play important roles in the pathogenesis of AD: APP, MAPT and APOE. Rare nonsynonymous coding variants in APP can cause presenile AD and APP codes for amyloid-beta precursor protein, which undergoes cleavage to form Aβ fragments and these aggregate to form the amyloid plaques which are a hallmark of AD [6].