Glioblastoma Multiforme (GBM) is among the most devastating cancers, with a median survival of approximately 1 year. 1 GBM presents unique challenges to therapy due to its location, aggressive biological behavior and diffuse infiltrative growth. Despite the development of new surgical and radiation techniques and the use of multiple antineoplastic drugs, a cure for malignant gliomas remains elusive. 2 The scarce efficacy of current treatments reflects the resistance of glioblastoma cells to cytotoxic agents in vitro. 3, 4 Moreover, the short interval for tumor recurrence in glioblastoma patients suggests that tumorigenic cells are able to overtake the treatments without major damage. The cancer stem cell hypothesis asserts that solid tumors are maintained exclusively by a rare fraction of cancer cells with stem cell properties. The existence of cancer stem cells was first proven in the context of acute myeloid leukemia. 5 More recently, this principle has also been extended to other tumors, such as breast and brain cancer. 6–8 Cancer stem cells have been reported to be the only tumorigenic population in GBM, their unlimited proliferative potential being required for tumor development and maintenance. 8 Thus, these cells should represent the primary therapeutic target in order to achieve complete eradication of the tumor. We isolated undifferentiated GBM cells from surgical specimens (Figure 1a, b) through mechanical dissociation of the tumor tissue and culture in a serum free medium supplemented with epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) as previously described. 7 Isolated cells were expanded and characterized both in vitro and in vivo. GBM-derived cell clones were able to grow in vitro in aggregates called tumor spheres and maintain an undifferentiated state, as indicated by morphology and expression of stem cell markers such as CD133 and nestin (not shown). The in vivo tumorigenic potential of GBM tumor spheres was assayed by intracranial or subcutaneous cell injection in immunocompromised mice. GBM stem cells were able to generate a tumor identical to the human tumor in terms of antigen expression and histological tissue organization (Figure 1c, d and data not shown). Altogether, these features of GBM stem cells indicate that they may provide a reliable in vitro and in vivo model for studying glioblastoma response to treatments. Therefore, we ought to investigate the effect of different chemotherapeutic agents on GBM stem cell survival and expansion.