The fEOAD represents 1–5% of all AD cases, and 40% fEOAD cases are associated with autosomal dominant mutations in PSEN1 on chromosome 14 (encoding presenilin 1), PSEN2 on chromosome 1 (encoding presenilin 2), and APP on chromosome 21 (encoding amyloid β precursor protein). The gene discussed is PSEN2; the disease is Alzheimer disease.