Cancer stem cells are cells within a tumor that are able to divide indefinitely and develop into other cell types. While they appear to make up only a small fraction of tumor cells, they are thought to be the reason why cancer persists, metastasizes and relapses. The nature and even the existence of cancer stem cells, however, remains a hotly debated topic. This gallery features four researchers at Children’s Hospital Boston who are investigating cancer stem cells, with a view to developing better cancer therapies. At left: A muscle-type tumor in a transgenic zebrafish, showing immature cancer cells, including cancer stem cells, in red.