The APOE e4 allele is found in the majority of patients with Alzheimer's disease. An individual who has tested positive for the APOE e4 allele has a 3- to 15-fold increased risk of eventually developing late-onset Alzheimer's disease. However, since not all individuals who have the APOE e4 eventually develop the disease, the presence of the APOE e4 allele is not by itself diagnostic of late-onset Alzheimer's disease in a symptomatic individual.