Several independent studies have provided strong evidence for the existence of cancer stem cells in some brain tumors, and potentially in skin and colon cancers as well. Like their powerful, healthy counterparts, the putative cancer stem cells have the ability to endlessly self-renew and produce progeny that can develop into all the different types of cells within a tumor. Some may even be resistant to many standard cancer therapies, and could be the cause of cancer relapse. But whether they actually exist in solid tumors is a controversial notion. Three studies published today won't end the debate, but they do lend greater credence to the theory, and could provide support for new approaches to cancer treatment.