The aforementioned publication describing microsatellites as EWS-FLI1 targets pointed out a requirement for minimal length of four GGAA repeats for binding. The rabbit pyrogenicity test can be used only when there is interference with the endotoxin test, which was the case here. In fact the pyrogenicity test encompasses all pyrogens and not only endotoxins. We applied the real volumes that were to be applied on patients and took the largest safety factor for acceptability of the phage cocktail. In theory, there is no need to ascertain the absence of pyrogens from products, which are not intravenously/parenterally administered. However, we worked with a product that during its production process was in close contact with bacteria and that by application to a burn wound could diffuse partially into the blood stream. Our study further indicates that a strong in vivo overrepresentation is observed for microsatellites containing between 9 and 17 repeats. In agreement with the hypothesis that such repeats play a role in EWS-FLI1-driven transcription regulation, we observe that a dramatic effect on expression of a reporter gene is indeed observed for this range of repeats both in heterologous 293T and Ewing cells. This is also in agreement with a recent study on NR0B1 showing that the level of expression of this gene in different Ewing cell lines is correlated to the number of GGAA repeats in its promoter. Yet, the precise mechanism underlying such binding needs further investigation. Cooperative binding or increased probability of binding due do the high local concentration of binding sites have been proposed. The DNA conformation, and in particular the DNA bending that has been previously shown to be crucial for ETS factors binding, may also be influenced by the number of GGAA repeats. Further EX 527 ChIP-Seq experiments are required to increase the depth of the analysis and evaluate in vivo the potential of EWS-FLI1 to bind different microsatellite sequences. In particular, this will enable to search for the presence in the vicinity of GGAA repeats of binding sites for specific transcription factors that may cooperate with EWS-FLI1 for binding. It will also be very informative to combine these EWS-FLI1 analyses with genome-wide studies of epigenetic landmarks since chromatin conformation may be crucial for EWS-FLI1 binding. Combining the ChIP strategy to global gene expression microarrays reveals that sites with long GGAA microsatellites are preferentially localized near EWS-FLI1 positively modulated genes. Several EWS-FLI1 modulated genes located in the vicinity of GGAA repeats can now be tested for their implication in Ewing sarcoma oncogenesis, such as the kinases DLG2 and VRK1, the latter being involved in cell cycle regulation possibly through the regulation of p53 function. Interestingly, EWS-FLI1 gene modulation via microsatellites targeting might be more general than suggested by the present analysis as a number of EWS-FLI1 up-regulated genes.