Familial AD (fAD) is caused by mutations in three genes: amyloid precursor protein (APP), presenilin 1 and presenilin 2 (PSEN1/PSEN2) (Goate et al., 1991; Levy-Lahad et al., 1995; Sherrington et al., 1995). This evidence concerns the gene PSEN1 and familial Alzheimer disease.