Antigens were retrieved by boiling samples for 12 minutes in citrate buffer, pH 6 (Invitrogen). cells that retain sensitivity to EWS-FLI1 inhibition. Inhibiting the EWS-FLI1 oncoprotein may prove to be an effective means of improving patient outcomes by targeting Ewing’s sarcoma stem cells that survive standard chemotherapy. == Introduction == Ewing sarcoma family tumors (ESFT) are the second most common bone tumor in children and young adults. ESFT initially responds quite well to FLJ31945 cytotoxic chemotherapy, but 30% of patients presenting with localized tumors develop recurrent disease and 6580% of patients with metastatic ESFT die within 5 years of diagnosis[1],[2],[3]. The cancer stem cell (CSC) hypothesis provides a framework for explaining the discrepancy between response of ESFT to therapy and the poor survival rate. The origin of CSCs remains controversial, but whether they are derived directly from normal tissue stem cells, or from differentiated cells that have acquired stem cell properties through genetic mutations, these cells, like normal stem cells, can undergo asymmetric division and are capable of self renewal as well as giving rise to a population of differentiated tumor cells. CSCs have been identified in many hematologic and solid tumors, including both acute and chronic leukemias, brain tumors, breast cancer, and colon cancer[4],[5],[6],[7]. CSCs are postulated to be resistant to standard cytotoxic chemotherapeutic agents both in vitro and in vivo, and this resistance is postulated to be the major cause of treatment failure, as the surviving reservoir of stem cells repopulates the tumor leading to PP1 relapse. Nevertheless, not every tumor type adheres to the stem cell model[8]. Thus, there is an urgent need to identify and fully characterize ESFT stem cells, and to develop therapeutic approaches to target these cells and improve the survival of patients with recurrent or metastatic disease. In 95% of cases, ESFT is associated with a translocation between the central exons of the EWS gene located on chromosome 22 and the central exons of anetsfamily gene; either FLI1 located on chromosome 11, or ERG located on chromosome 21[9]. The resulting fusion protein acts as an aberrant transcription factor regulating genes involved in transformation. The EWS-FLI1 fusion protein is an excellent candidate for targeted therapy as its expression is limited to tumor cells and is crucial for initiation and maintenance of the tumor. Reducing EWS-FLI1 expression using antisense oligonucleotides or siRNA in cell lines results in decreased tumorigenicity bothin vitroandin vivo[10],[11],[12]. Moreover, transduction of mesenchymal stem cells with EWS-FLI1 causes the development of tumors with an ESFT phenotype[13]. The mechanism by which EWS-FLI1 mediates neoplastic transformation is poorly understood. EWS-FLI1 has transcriptional regulatory activity, and a number of target genes have been identified that may play a role in neoplastic transformation[14],[15],[16],[17]. In addition to this activity, RNA helicase A (RHA) physically interacts with EWS-FLI1 and modulates oncogenesis, suggesting that this interaction is a promising therapeutic target[18]. We have developed PP1 a novel small molecule, YK-4-279, that inhibits the EWS-FLI1/RHA interaction, inducing apoptosis in ESFT cell lines and xenografts[19]. Aldehyde dehydrogenase (ALDH) has been proposed to be a marker of both normal and cancer stem cells[20]and has been used to identify CSC from colon, breast, and lung cancers, among others[21],[22],[23]. We have identified a CSC population in ESFT cell lines and xenografts based on high expression of ALDH. These cells fulfill thein vitroandin vivocriteria for stem cell activity including the ability to reconstitute PP1 a heterogeneous population, sphere- and colony-forming activity, and the ability to form tumors.