Ewing's sarcoma is a malignant bone-associated tumor of children and young 
adults. Most cases of Ewing's sarcoma express the EWS/FLI fusion protein. 
EWS/FLI functions as an aberrant ETS-type transcription factor and serves as the 
master regulator of Ewing's sarcoma-transformed phenotype. We recently showed 
that EWS/FLI regulates one of its key targets, NR0B1, through a 
GGAA-microsatellite in its promoter. Whether other critical EWS/FLI targets are 
also regulated by GGAA-microsatellites was unknown. In this study, we combined 
transcriptional analysis, whole genome localization data, and RNA interference 
knockdown to identify glutathione S-transferase M4 (GSTM4) as a critical EWS/FLI 
target gene in Ewing's sarcoma. We found that EWS/FLI directly binds the GSTM4 
promoter, and regulates GSTM4 expression through a GGAA-microsatellite in its 
promoter. Reduction of GSTM4 levels caused a loss of oncogenic transformation. 
Furthermore, reduction of GSTM4 resulted in an increased sensitivity of Ewing's 
sarcoma cells to chemotherapeutic agents, suggesting a role for this protein in 
drug resistance. Consistent with this hypothesis, patients with Ewing's sarcoma 
whose tumors had higher levels of GSTM4 expression had worse outcomes than those 
with lower expression levels. These data show that GSTM4 contributes to the 
cancerous behavior of Ewing's sarcoma and define a wider role for 
GGAA-microsatellites in EWS/FLI function than previously appreciated. These data 
also suggest a novel therapeutic resistance mechanism, in which the central 
oncogenic abnormality directly regulates a resistance gene.