Three genes, presenilin 1 (PSEN1), presenilin 2 (PSEN2), and amyloid precursor protein (APP), have known pathogenic mutations that are strongly associated with the development of early-onset AD (Fig. 2); although mutations in additional genes may be involved in the pathology of the disease as well [65, 66]. This evidence concerns the gene PSEN2 and Alzheimer disease.