PSEN1 and Alzheimer disease: Two different forms of AD have been described: the inherited form called “early-onset familial AD” (FAD) is caused by rare genetic variations in genes encoding amyloid precursor protein (APP), presenilin 1 (PSEN1), and 2 (PSEN2) involved in β-amyloid (Aβ) processing and which affect the intracellular trafficking of Aβ or APP (26, 27).