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