Genetic causes, like mutations in amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2), have been implicated in early-onset AD, while the apolipoprotein E epsilon 4 (APOE ε4) allele is the strongest risk factor for late-onset AD. Here, PSEN1 is linked to Alzheimer disease.