PSEN1 and early-onset autosomal dominant Alzheimer disease: A minority of Alzheimer’s disease cases are associated with genetic mutations in the genes encoding APP (amyloid precursor protein), PSEN1 (presenilin 1), and PSEN2 (presenilin 2), and they are typically linked to early-onset Alzheimer’s disease (EOAD), in which symptoms generally appear before the age of 65.