The causes of Alzheimer’s disease are not fully understood. In the small number affected by the early-onset form, a genetic mutation is usually the cause and this disease tends to cluster within families. The more common late-onset form occurs due to changes in the brain that occur over decades. These changes likely occur due to a combination of genetic, environmental and lifestyle factors. One gene in particular, APOE, is the biggest genetic risk factor linked to the late-onset form.