In Alzheimer's disease (AD), characterized by the presence of amyloid-beta (Aβ) plaques and tau protein tangles, critical genetic factors include the amyloid precursor protein (APP), presenilin 1 (PSEN1), presenilin 2 (PSEN2), and apolipoprotein E (APOE). The gene discussed is PSEN2; the disease is early-onset autosomal dominant Alzheimer disease.