A number of solid tumors, such as alveolar rhabdomyosarcoma, synovial sarcoma, 
and myxoid liposarcoma, are associated with recurrent translocation events that 
encode fusion proteins. Ewing's sarcoma is a pediatric tumor that serves as a 
prototype for this tumor class. Ewing's sarcomas usually harbor the 
(11;22)(q24;q12) translocation. The t(11;22) encodes the EWS/FLI fusion 
oncoprotein. EWS/FLI functions as an aberrant transcription factor, but the key 
target genes that are involved in oncogenesis are largely unknown. Although some 
target genes have been defined, many of these have been identified in 
heterologous model systems with uncertain relevance to the human disease. To 
understand the function of EWS/FLI and its targets in a more clinically relevant 
system, we used retroviral-mediated RNAi to "knock-down" the fusion protein in 
patient-derived Ewing's sarcoma cell lines. By combining transcriptional 
profiling data from three of these lines, we identified a conserved 
transcriptional response to EWS/FLI. The gene that was most reproducibly 
up-regulated by EWS/FLI was NR0B1. NR0B1 is a developmentally important orphan 
nuclear receptor with no previously defined role in oncogenesis. We validated 
NR0B1 as an EWS/FLI-dysregulated gene and confirmed its expression in primary 
human tumor samples. Functional studies revealed that ongoing NR0B1 expression 
is required for the transformed phenotype of Ewing's sarcoma. These studies 
define a new role for NR0B1 in oncogenic transformation and emphasize the 
utility of analyzing the function of EWS/FLI in Ewing's sarcoma cells.