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Stem cells and cancer are inextricably linked; the process of carcinogenesis initially affects normal stem cells or their closely related progenitors and then, at some point, neoplastic stem cells are generated that propagate and ultimately maintain the process. Many, if not all, cancers contain a minority population of self-renewing stem cells, "cancer stem cells", that are entirely responsible for sustaining the tumour and for giving rise to proliferating but progressively differentiating cells that contribute to the cellular heterogeneity typical of many solid tumours. Thus, the bulk of the tumour is often not the clinical problem, and so the identification of cancer stem cells and the factors that regulate their behaviour are likely to have an enormous bearing on the way that we treat neoplastic disease in the future. This review summarises (1) our knowledge of the origins of some cancers from normal stem cells and (2) the evidence for the existence of cancer stem cells; it also illustrates some of the stem cell renewal pathways that are frequently aberrant in cancer and that may represent druggable targets.

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