Several of these studies have shown that the target gene repertoire of EWS-FLI-1 varies according to the host cell type. To determine whether EWS-FLI-1 and other ESFT-associated fusion proteins trigger similar responses in cells from which ESFT are believed to originate, we stably introduced EWS-FLI1, EWS-ERG and FUS-ERG into MPC and addressed the corresponding transcription profile changes. We compared these changes to those induced by FLI1 and ERG1 alone as well as to those induced by an isoform of FUS-ERG associated with acute myeloid leukemia but not ESFT. Our results show that MPCs display differential permissiveness for EWS-FLI-1, EWS-ERG and FUS-ERG and that among the gene expression changes induced by the three fusion proteins only a limited fraction are shared. One of the genes observed to be induced by all three fusion proteins was IGF1. In the present work we provide evidence that IGF1 is a direct target gene of ESFT fusion proteins. The distinguishing feature of Ewing��s sarcoma is the expression of an aberrant transcription factor encoded by a fusion gene resulting from a Pancuronium dibromide non-random chromosomal translocation. In all cases the fusion protein is composed of the amino terminal portion of a TET family member that provides a potent transactivating domain and the DNA binding domain of one of several possible ets family members. In more than 99% of cases, the TET family member is EWS and in 85�C90% of cases the ets family member is FLI-1. EWS is fused to ERG in 5�C10% of cases whereas FUS-ERG is found in less than 1% of cases. The difference in frequency of association of the fusion proteins with ESFT is currently unexplained, and could conceivably reflect the relative frequency of the corresponding chromosomal breaks. However, our present observations using mouse MPCs suggest that primary mesenchymal stem cells display a markedly different degree of permissiveness for the three fusion proteins. Whereas expression of EWS-FLI-1 was tolerated in all of the cell batches tested, expression of EWS-ERG was restricted to a fraction of the batches while stable expression of FUS-ERG could not be achieved. The observed differential permissiveness correlates with the relative frequency at which each fusion accompanies ESFT cases, suggesting distinct windows of opportunity for the different fusion proteins to display their putative transforming properties in MPCs. Mechanisms whereby MPCs restrict expression of EWS-ERG and FUS-ERG remain to be elucidated. Whereas in some cases the specific RNA was not detectable or appeared degraded, in Epimedoside-A others protein expression could not be detected despite maintenance of transcripts of appropriate length. It is conceivable that discrete stages of MPC differentiation may account for the observed differences in permissiveness. Alternatively, MPCs may be composed of functionally heterogeneous cell subsets that cannot be distinguished on the basis of the restricted number of phenotypic markers used to characterize them. A plausible scenario may be that a majority of these putative subsets display a milieu that is favourable for EWS-FLI-1 expression and function, whereas only rare subsets may tolerate expression of FUS-ERG. Thus, the composition or differentiation stage of MPC populations may determine whether or not expression of ESFT-associated fusion proteins other than EWS-FLI-1 may be sustained. The observed difference in permissiveness for fusion protein expression could not be attributed to functional differences among MPC batches because the same batches were used for expression of all of the contructs.