PSEN2 and Alzheimer disease: Unlike the multifactorial etiology of late-onset sporadic AD, autosomal dominant forms (which are relatively rare) have early onset, occurring before 65 years (presenile AD), and are strongly associated with mutations of the amyloid precursor protein (APP), presenilin 1, or presenilin 2 genes, which are identified in 70% of cases, and with a dominant, autosomal inheritance pattern.5