Researchers have known for many years that Alzheimer’s disease involves two proteins called amyloid beta and tau, and that interaction between them drives the spread of neurodegeneration. What hasn’t been clear is where and how those interactions occur over time as disease progresses. In a study published April 19 in the journal Neuron, neurologist William Seeley, MD, and his colleagues at UC San Francisco and in South Korea identified two key moments in the natural history of the disease.