AD is a complex multifactorial condition with a broad spectrum of manifestations, ranging from early-onset cases – frequently associated with rare autosomal dominant mutations in APP (amyloid beta precursor protein), PSEN1 (presenilin 1) and PSEN2 (presenilin 2) genes – to late-onset sporadic cases, which are associated with common alleles of small effect sizes that in aggregation contribute to genetic susceptibility to AD [274, 275]. Here, PSEN1 is linked to Alzheimer disease.