Scientists discovered a mysterious RNA in breast cancer that led them to discover an entire hidden class of cancer-specific RNA molecules. These molecules are found in dozens of tumor types and form unique molecular signatures that allow cancer type and subtype to be identified with high precision. Some of these RNA molecules have a direct effect on tumor growth and its spread to other tissues. Since many of these RNA molecules pass into the bloodstream, they can be detected with a simple blood test. Such a test could be used to monitor how patients respond to treatment and to predict their survival. These findings represent a significant advance in the understanding of the molecular mechanisms of cancer and in the possibilities of its diagnosis and monitoring.