One of the great misconceptions about cancer is that, since tumors originate from normal cells, they are able to disguise themselves from the immune system—lurking undetected and unopposed as they divide and proliferate. In reality, the immune system is no passive observer when it comes to cancer. Evidence is mounting that many tumors undergo almost constant immune attack. But just how these attacks are initiated and what their effect is on different tumor types has remained largely unexplored. Until now.