For some time, we've known that it's rare to see people with both cancer and Alzheimer's disease. Now, scientists believe they may have found why, discovering a molecule in cancer cells that clears problem proteins from the brain. In Alzheimer’s disease (AD), the build-up of misfolded amyloid-beta plaques and tau tangles destroys the brain's cells and leads to cognitive decline. So much so that scientists have found that some cancer treatments have delivered positive results in slowing cognitive decline. While many scientists have believed for some time there's a mysterious relationship between AD and cancer, disease pathology has made it difficult to understand.