This early-onset AD form is caused by mutations in three genes that encode specific proteins, i.e., β-Amyloid precursor protein (APP) on chromosome 21, presenilin-1 (PSEN1) on chromosome 14 and presenilin-2 (PSEN2) on chromosome 1 with the latter covering alone about 1% of cases [59]. The gene discussed is PSEN1; the disease is Alzheimer disease.