At the heart of every tumor, some researchers believe, lie a handful of aberrant stem cells that maintain the malignant tissue.

The idea, if right, could explain why tumors often regenerate even after being almost destroyed by anticancer drugs. It also points to a different strategy for developing anticancer drugs, suggesting they should be selected for lethality to cancer stem cells and not, as at present, for their ability to kill just any cells and shrink tumors.