PSEN2 and Alzheimer disease: For example, the mutations that cause autosomal dominant familial Alzheimer’s disease (FAD) arise in only three genes: the amyloid precursor protein (APP) gene, the presenilin 1 (PSEN1) gene, and the presenilin 2 (PSEN2) gene, all three of which encode proteins involved in the production of the Aβ peptide, which is the main pathogenic molecule of AD (Goate and Hardy, 2012; Hardy, 2017).