A definitive diagnosis of Alzheimer’s disease (AD) was once only possible after someone had died, but recent biomarker studies have led to the development of imaging and spinal fluid tests for those still living. However, the tests can only monitor severe disease, differentiating advanced AD from related disorders. Reporting in ACS Chemical Neuroscience, researchers have now identified a biomarker that could help physicians diagnose AD earlier, as a patient transitions into mild cognitive impairment (MCI).