Familial AD which accounts for 2% of all cases and may have a disease onset as early as 40–50 years, is caused by mutations in the amyloid precursor protein (APP) gene and presenilin 1–2 genes prevalently (Karlinsky et al., 1992; Levy-Lahad et al., 1995; Sherrington et al., 1995). The gene discussed is APP; the disease is Alzheimer disease.