PSEN2 and Alzheimer disease: In cases of specific familiar forms of AD, which are recurrent among family members and are characterized by early onset (i.e., age < 65), disease causing mutations in specific genes have been identified, namely amyloid precursor protein (APP), Presenilin 1 PSEN1 and Presenilin 2 PSEN2 (Piaceri et al., 2013).