APP and Alzheimer disease: Missense mutations in three different genes encoding amyloid-β precursor protein [1] (APP, [MIM 104760]), presenilin 1 [2] (PSEN1, [MIM 104311]) and presenilin 2 [3] (PSEN2, [MIM 600759]) are recognized to cause familial early-onset Alzheimer disease (EO-AD; [MIM 104300]).