The vast majority of AD is sporadic (SAD), but mutations in the amyloid precursor protein (APP) and in presenilin-1 (PSEN1) and -2 (PSEN2), which are components of the γ-secretase complex that processes APP to produce amyloid-β (Aβ), have been identified in familial forms of AD (FAD), which have an earlier age of onset [2–4]. The gene discussed is PSEN2; the disease is Alzheimer disease.