PSEN2 and familial Alzheimer disease: SAD accounts for more than 99% of total cases and is characterized by a late-onset of symptomatology, whereas FAD is an early-onset form of the disease associated with mutations in three genes encoding for the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2) transmitted in an autosomal dominant pattern (Goate et al., 1991; Levy-Lahad et al., 1995; Rogaev et al., 1995; Sherrington et al., 1995).