Identification of genetic variants in the APP, PSEN1 and PSEN2 genes associated with familial AD, were found to favor the longer Aß42 peptide and enhance its production (Chartier-Harlin et al., 1991; Goate et al., 1991; Murrell et al., 1991; Citron et al., 1992; Hendriks et al., 1992; Mullan et al., 1992; Cai et al., 1993; Scheuner et al., 1996). Here, APP is linked to Alzheimer disease.