APOE and Alzheimer disease: For example, early-onset AD has been linked to mutations in the genes encoding amyloid precursor protein (APP) and the presenilins (PS-1 and PS-2) involved in amyloid-β processing [3,4], while the ε4 allele of the apolipoprotein E gene has been identified as a risk factor for late-onset AD [8,9,10].