Early onset Alzheimer Disease (AD) is caused by gene mutations that lead to abnormal processing of amyloid precursor protein (APP). A chromosome 21 mutation leads to the formation of abnormal amyloid precursor protein (APP). Presenilin genes on chromosomes 1 and 14 encode proteins that play a role in APP breakdown, and can also lead to an inherited form of the disease. Test for these mutations are available…but would you want to know?