Alzheimer disease is characterized by progressive loss of mental abilities (dementia). It is the most common type of dementia. It usually occurs after 60 years of age. The exact cause of Alzheimer disease is not known. Several factors probably play a role, including a specific allele of a gene called apolipoprotein E (apoE ε4 allele), high blood pressure, and high cholesterol level. However, how these factors relate to each other in increasing the risk for Alzheimer disease has not been clear.