APP and Alzheimer disease: This early-onset form of Alzheimer’s disease is rare and mutations typically found in the amyloid precursor protein (APP) and presenilin (PSEN) genes exacerbate the formation of the neurotoxic and pathologically related peptide, amyloid-β42 (Goate et al. 1991; Sherrington et al. 1995; Pimplikar 2009).