Cancer stem cell research is a focus for more and more cancer biologists and evidence of involvement incancer devleopment is becoming more abundant. Earlier studies indicated cancer stem cells to be rare asdetermined by the standard xenotransplantation assay using SCID mice in vivo. However, recent studies haveshown that syngeneic transplantation of mouse tumors or modifications to the xenotransplantation assay caneffectively improve the accuracy of detection, with stem cells being more abundant than hitherto thought.Furthermore, to estimate frequency correctly, it is necessary to considerate cancer stem cell subsets with differingcapacities for tumorigenesis.
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