APP and familial Alzheimer disease: Genetic linkage analysis of familial Alzheimer’s disease (FAD) identified amyloid precursor protein (APP) (Chartier-Harlin et al. 1991; Goate et al. 1991; St George-Hyslop et al. 1987) and presenilins i.e. PSEN1 (Citron et al. 1997; Sherrington et al. 1995) and PSEN2 (Levy-Lahad et al. 1995) as the causative genes in FAD.