A: Is tumor growth sustained by rare cancer stem cells or dominant clones

A key issue for cancer biology and therapy is whether the relentless growth of a tumor is driven by a substantial proportion of its cells or exclusively by a rare subpopulation, commonly termed ‘‘cancer stem cells.’ ’ Support for the cancer stem cell model has been stimulated by experiments in which human tumor cells were transplanted into immunodeficient mice. Most notably, in human acute myeloid leukemia, only a minute proportion of the cells, displaying a defined pheno-type, could seed leukemia in mice. Xenotransplantation, however, may fail to reveal many tumor growth–sustaining cells because the foreign microenvironment precludes essen-tial interactions with support cells. In studies that instead have transplanted mouse leukemias and lymphomas into syngeneic animals, most of the tumors seem to be maintained by the dominant cell population, and only a few types of mouse leukemia seem to be sustained by a minor tumor growth–sustaining subpopulation. The collective evidence suggests that various tumors may span the spectrum between the extremes represented by the two models. If tumor growth can indeed be sustained either by rare cancer stem cells or dominant clones or both, as current evidence suggests, curative therapy for many types of tumors will most likely require targeting all the tumor cell populations. [Cancer Re