Glioblastoma multiforme is the most common primary malignant brain tumor, with a median survival of about one year. The poor prognosis is due to therapeutic resistance and tumor recurrence after surgical removal. Using a genetically engineered mouse model of glioma, the authors identify a subset of endogenous tumor cells that are responsible for new tumor growth after the drug temozolomide (TMZ) transiently arrests tumor growth. A nestin-ATK-IRES-GFP (Nes-ATK-GFP) transgene labels quiescent subventricular zone adult neural stem cells and a subset of endogenous glioma tumor cells. Pulse-chase experiments demonstrate that tumor re-growth originates from the Nes-ATK-GFP subpopulation. Ablation of the GFP+ cells with chronic ganciclovir (GCV) administration significantly arrested tumor growth, and combined TMZ and GCV treatment impeded tumor development. This study identifies a relatively quiescent subset of endogenous glioma cells, similar to cancer stem cells, that sustain long-term tumor growth through the production of highly proliferative cells.Glioblastoma multiforme is the most common primary malignant brain tumor, with a median survival of about one year. The poor prognosis is due to therapeutic resistance and tumor recurrence after surgical removal. Using a genetically engineered mouse model of glioma, the authors identify a subset of endogenous tumor cells that are responsible for new tumor growth after the drug temozolomide (TMZ) transiently arrests tumor growth. A nestin-ATK-IRES-GFP (Nes-ATK-GFP) transgene labels quiescent subventricular zone adult neural stem cells and a subset of endogenous glioma tumor cells. Pulse-chase experiments demonstrate that tumor re-growth originates from the Nes-ATK-GFP subpopulation. Ablation of the GFP+ cells with chronic ganciclovir (GCV) administration significantly arrested tumor growth, and combined TMZ and GCV treatment impeded tumor development. This study identifies a relatively quiescent subset of endogenous glioma cells, similar to cancer stem cells, that sustain long-term tumor growth through the production of highly proliferative cells.