EWS/FLI and other EWS/ets chimeric transcription factors play a central role in the biology of the Ewing family tumors. As with many oncogenes, EWS/FLI biologic activity can be demonstrated in a limited range of cellular contexts. To investigate the causes of this restriction, we demonstrate that two immortalized fibroblast lines resistant to EWS/FLI transformation, Rat1 and Yal7, express stable levels of EWS/FLI protein. Despite their resistance to EWS/FLI, Rat1 and Yal7 can be transformed by the potent EWS/FLI downstream mediator PDGF-C. In contrast to NIH3T3, the EWS/FLI resistant lines show no upregulation of PDGF-C in response to EWS/FLI, demonstrating differential EWS/FLI function in different cellular backgrounds. This phenomenon of differential function can also be demonstrated for several other NIH3T3 targets of EWS/FLI. Despite the correlation between anchorage-independent growth and PDGF-C induction, PDGF-C does not fully reproduce all aspects of the EWS/FLI phenotype in NIH3T3 cells. These results further point to the importance of PDGF-C in mediating EWS/FLI in vitro transformation and suggest caution in assuming that a transcription factor will produce identical effects in different cellular backgrounds. © 2003 Elsevier Inc. All rights reserved.