PSEN2 and Alzheimer disease: Familial AD (FAD), representing less than 2% of the total, is an autosomal dominant disorder with onset before age 65 years, caused by mutations of the APP gene on chromosome 21 [4], and, less frequently, mutations in the highly homologous presenilin 1 (PSEN1) on chromosome 14, and presenilin 2 (PSEN2) on chromosome 1 [5–7].