PSEN1 and Alzheimer disease: The familial AD accounts for 5% of AD cases and usually develops before the age of 65 (some patients begin in their 30s or 40s), mainly associated with mutations in APP gene on chromosome 21, presenilin-1 (PSEN1) on chromosome 14, and presenilin-2 (PSEN2) gene on chromosome 1 [36,38].