Alzheimer's disease is characterized by extracellular amyloid aggregates of toxic 40- or 42-amino-acid long amyloid-β (Aβ40 or Aβ42) peptides, which are generated from amyloid precursor protein (APP) that is abnormally cleaved by α- and γ-secretases. The gene discussed is APP; the disease is early-onset autosomal dominant Alzheimer disease.