The fundamental role of Aβ in AD is derived from studies of fAD, which accounts for 1–5% of patients with AD [117], who have autosomal dominant mutations or duplications in the amyloid precursor protein (APP) or mutations in the presenilin-1 (PSEN1) and presenilin-2 (PSEN2) genes [118,119]. The gene discussed is PSEN2; the disease is familial Alzheimer disease.