The principal genetic causes of AD are mutations in different genes, such as APP (Goate et al., 1991), γ-secretase proteins presenilin-1 and presenilin-2 (PSEN1-2) (Scheuner et al., 1996), which result in the production of aggregation-prone Aβ peptides, called oligomers. The gene discussed is PSEN1; the disease is Alzheimer disease.