PSEN2 and Alzheimer disease: There are three genes identified for familial AD, including Amyloid Beta Precursor Protein (APP) [39], Presenilin 1 (PSEN1), and Presenilin 2 (PSEN2), but only one gene, Apolipoprotein E allele ε4 (APOEε4), is associated with the sporadic forms of AD [40,41].