PROJECT SUMMARY Glioblastomas are the most common and aggressive primary brain tumor in adults, with a median survival of only fourteen months with the best available treatments. Much needed novel therapies may arise from increased appreciation of the biological and molecular properties of subset of tumor cells which can self-renew and recapitulate the parental tumor. These cancer stem cells remain controversial due to the evolving understanding of their nature: however, a number of reports have demonstrated that glioblastomas contain cancer stem cells and that these glioma stem cells contribute to therapeutic resistance and tumor angiogenesis. We now demonstrate that the cell survival regulator A20/Tumor Necrosis Factor a Inducible Protein 3 is a glioma stem cell target which contributes to glioma growth. Although very limited and often contradictory data exists regarding the expression and function of A20 in brain tumors, we find that GSCs consistently express elevated levels of A20 in comparison to non-stem glioma cells. Targeting the expression of A20 in GSCs reduces their growth in association with increased cell cycle arrest, elevated apoptosis, and decreased self-renewal. Targeting A20 in GSCs increased the survival of mice bearing human glioma xenografts, and analysis of a glioma expression database indicates that increased A20 mRNA correlates with poor glioma patient survival. Based on this background, we hypothesize that A20 promotes glioma growth and recurrence due, in part, to maintenance of a cancer stem cell phenotype. We propose to elucidate the molecular and biological role of A20 in glioma stem cell biology in an effort to determine novel targets for glioma patient therapies.