In Alzheimer’s disease (AD), mutations in one of three genes—the amyloid precursor protein (APP) gene and the presenilin protein 1 and 2 genes (PS1 and PS2), which encode subunits of gamma-secretase—contribute to the development of an early onset disease [155], and the allele E4 of the APOE gene increases considerably the risk of development of AD [156]. Here, APP is linked to Alzheimer disease.