In this cohort study, CSF amyloid decreased toward abnormal levels in controls, declined further in the amyloid-positive cognitively unimpaired group , and was concurrent with decline of delayed recall; CSF amyloid stabilized in those with amyloid-positive MCI and dementia, while tau markers became increased (ie, more abnormal) in the amyloid-positive cognitively unimpaired and amyloid-positive MCI groups, suggesting that increase in CSF tau requires abnormal amyloid. The gene discussed is MAPT; the disease is dementia.