A new family of presenilin genes involved in the pathogenesis of early-onset autosomal dominant Alzheimer's disease (AD) has been identified recently. Mutations in presenilin-1 and presenilin-2 genes have a full penetration and lead to AD. The presenilins are transmembrane proteins localized mainly within endoplasmatic reticulum and Golgi systems. Their biological function remains unknown. It has been suggested that the presenilins may be important for intracellular processes of protein trafficking and processing. Presenilin mutations play a direct role in the beta-amyloid precursor protein metabolism and cause increased production of amyloidogenic A beta 42(43) peptide.