Considering the defect at the genetic level, AD results from the abnormality in the expression of five genes: Apolipoprotein E (APOE), Amyloid Precursor Protein (APP), Beta-site Amyloid precursor protein Cleaving Enzyme 1 (BACE1), Presenilin 1 and 2 (PSEN-1, and PSEN-2). Here, PSEN1 is linked to Alzheimer disease.