Clinical signs of Alzheimer's disease are characterized by progressive cognitive deterioration, together with declining activities of daily living and by neuropsychiatric symptoms or behavioral changes. The most striking early symptom is memory loss (amnesia), usually manifest as minor forgetfulness that becomes steadily denser with illness progression, with relative preservation of older memories.

It is the most common type of dementia. Plaques which contain misfolded peptides called amyloid beta (Aβ) are formed in the brain many years before the clinical signs of Alzheimer's are observed. Together, these plaques and neurofibrillary tangles form the pathological hallmarks of the disease. These features can only be discovered at autopsy and help to confirm the clinical diagnosis. Medications can help reduce the symptoms of the disease, but they cannot change the course of the underlying pathology.

The ultimate cause of Alzheimer's is unknown. Genetic factors are clearly indicated as evidenced by dominant mutations in three different genes have been identified that account for the small number of cases of familial, early-onset AD. For the more common form of late onset AD (LOAD), ApoE is the only clearly established susceptibility gene. All four genes can contain mutations or variants that confer increased risk for AD, but account for only 30% of the genetic picture of AD. These four genes have in common the fact that mutations in each lead to the excessive accumulation in the brain of Aβ, the main component of the senile plaques that litter the brains of AD patients.