The most common gene associated with early-onset AD is presenilin 1 on chromosome 14 which accounts for up to 70% of those cases. Presenilin 2 and amyloid precursor protein (APP) gene mutations account for a smaller fraction of early-onset AD. Microtubule-associated protein tau (MAPT) gene mutations are associated with frontotemporal dementia. APOE e4 alleles can increase the risk of late-onset AD by 5-15 fold.