Early Onset AD (EOAD) refers to familial forms associated with one of the three known autosomal dominant mutations, Amyloid Precursor Protein (APP), Presenilin 1 (PSEN1), or Presenilin 2 (PSEN2), and comprises about 5% to 6% of cases, with subjects developing symptoms before reaching 65 years of age. Here, PSEN1 is linked to Alzheimer disease.