Itis mainly characterized by two neuropathological featurestheaccumulation of extracellular amyloid β (Aβ) protein plaquesand neurofibrillary tangles primarily composed of hyperphosphorylatedTau protein., Majority of cases of AD areknown to be sporadic in nature; however, a small percentage of casesare familial (known as familial AD or FAD), caused by dominant autosomalmutations found in one of three genes: presenilin-1 (PSEN1), presenilin-2 (PSEN2), and amyloid precursor protein(APP)., Here, PSEN2 is linked to familial Alzheimer disease.