Cancer, stem cells and cancer stem cells: old ideas, new developments.

It has been suggested that, at least in some forms of cancer, a sub-population of slow-cycling, therapy-resistant cancer stem cells exists that has the ability to reconstitute the tumor in its entirety. If true, this model implies that conventional therapies based on targeting highly cycling cells within the tumor will leave the slow-cycling stem cell population intact, giving them the opportunity to reinitiate the tumor at a later date. This review discusses the evidence for this model and the likely implications for cancer treatment.