In this contest, mutation in the genes APP, presenilin 1 (PSEN1), and presenilin 2 (PSEN2) accounts for most cases of the familial (or early-onset) form of AD, by increasing the production and aggregation of Aβ and amyloid plaque formation (Tanzi and Bertram, 2005). Here, PSEN1 is linked to Alzheimer disease.