Although EWS-FLI-1 has the ability to directly modulate the expression of a broad repertoire of target genes, including induction and repression of oncogenes and tumor suppressor genes, respectively, these mechanisms do not provide the full explanation for ESFT pathogenesis. Based on our recent observations that miRNA-145 repression underlies the emergence of ESFT CSC, we compared the miRNA expression profiles of MSCs and ESFT cell lines to identify miRNAs that may be implicated in ESFT pathogenesis and that may provide potential therapeutic targets. Our observations indicate that ESFT display concomitant induction of the oncogenic miRNA 17�C92 cluster and repression of the entire let-7 tumor suppressor family. We show the let-7 family member let-7a to be a direct EWS-FLI-1 target gene, whose in vivo repression promotes ESFT cell tumorigenicity via induction of its target gene HMGA2. More importantly, we demonstrate that systemic delivery of synthetic let-7a significantly ALK5 Inhibitor II ALK inhibitor decreases tumor growth in vivo, We have previously identified miRNA-145 as a direct EWSFLI- 1 target gene, whose repression is implicated in ESFT development, suggesting that other miRNAs may be involved in the pathogenesis of these tumors. Using miRNA array profiling we uncovered a limited number of differentially expressed miRNA families in ESFT cells. Among induced miRNAs, we found the oncogenic miRNA 17�C92 cluster and its paralogs miRNA106a/b, whereas repressed miRNAs included miRNA 100, 125b as well as the entire let-7 family. Interestingly, the miRNA 17�C92 cluster has been reported to be directly induced by c-Myc, a known EWSFLI- 1 target gene, suggesting that this cluster may be indirectly modulated by EWS-FLI-1 through c-Myc induction. Among the let-7 miRNA family we focused on let-7a because of its reported functional role in diverse cancer types. Let-7a repression has been observed in diverse malignant tumor types, including a variety of sarcomas and carcinomas. Let-7a down-regulation is mediated by several mechanisms including Lin28-dependent AB1010 VEGFR/PDGFR inhibitor degradation and myc-dependent transcriptional repression. In ESFT, we have shown that direct EWS-FLI-1-mediated repression provides a novel regulatory mechanism of let-7a expression. Given its role as an inhibitor of differentiation, let-7a repression may participate in early EWS-FLI-1-mediated transformation, by enhancing primary cell permissiveness for EWS-FLI-1 expression and function, as well as in subsequent ESFT CSC maintenance.