PSEN2 and familial Alzheimer disease: Although the majority of cases of AD are sporadic, a small percentage of cases are familial (known as Familial AD or FAD), caused by dominant autosomal mutations found in one of three genes: amyloid precursor protein (APP), presenilin-1 (PSEN1), and presenilin-2 (PSEN2) (Bertram and Tanzi, 2012[6]; Zhang et al., 2020[111]).