GTICs were first isolated based on the expression of CD133 and thus CD133+ thought to unequivocally label glioma cells with stem cell properties26. However, CD133− glioma cells presenting stem cell and GTIC properties have also been reported2728 and CD133+ cells found in a variety of normal tissues other than tumours29. Nevertheless, we sought to investigate the role of putative GTIC surface markers for enriching cancer populations with stem cells properties. On the basis of our flow cytometry results (Fig. 1a), we first separated CD133+ and CD133− cells and performed in vitro single-cell tumour forming assays. To avoid limiting our analyses to a single marker, we also sorted out CD15+ and CD15− cells as well as CXCR4+ and CXCR4− populations. All different cell populations demonstrated comparable colony forming potential (Supplementary Fig. 2g). These results are in agreement with the notion that GTICs are very heterogeneous. Indeed, a universally accepted panel of markers for the characterization and isolation of GTICs is yet to be reported30. Variability in surface marker expression in cancer cells bearing stem cell properties is not exclusive to gliomas
