PSEN1 and familial Alzheimer disease: The genes primarily involved in AD are Amyloid Precursor Protein (APP), Presenilin 1 (PSEN1), and Presenilin 2 (PSEN2), which are associated with early-onset familial AD (fAD), variations in apolipoprotein E (APOE) gene increase the risk of late-onset sporadic AD (sAD) [10].