Mutations in the genes for the amyloid-β precursor protein (APP) and the presenilins (PSEN1 and PSEN2) cause early-onset, dominantly inherited forms of Alzheimer’s disease (AD), whereas mutations in the MAPT gene mainly lead to frontotemporal dementia (FTD). The gene discussed is PSEN2; the disease is early-onset autosomal dominant Alzheimer disease.