Cancer research has been diversifying rapidly in recent years, the result of new technology and new knowledge. The International Herald Tribune reports on the latest new direction: "Within each tumor [lurks] a small population of elusive, highly potent cells that drive the tumor's growth ... In the past two years, cancer stem cells have gone from a theory on the fringes of biology to an idea that is attracting money and talent in cancer research. ... Most treatments today are judged by their ability to shrink tumors, but the new results suggest the size of the tumor is all but irrelevant: If doctors can kill the stem cells, the tumor is doomed, but if the stem cells survive, it will be back." Knowledge empowers scientists to develop targeted, effective therapies - we can hope this will happen here.