Chromosomal translocation t(11; 22)(q24; q12) is detected in approximately 90% of Ewing's family tumors (EFTs) including Ewing's sarcoma and primitive neuroectodermal tumor. This results in the formation of the EWS-Fli1 fusion gene, which produces EWS-Fli1 fusion protein. This chimerical gene product acts as an aberrant transcriptional activator, which may be responsible for the tumorigenesis of EFTs. We have previously reported that cyclin E expression was upregulated in EFT cells and in EWS-Fli1 transformed fibroblastic cells. However, the mechanism of the overexpression of cyclin E by EWS-Fli1 is still unknown. In our study, we investigated the mechanism of transactivation of the cyclin E gene in EFT cells. We found that EWS-Fli1 enhanced the activity of the cyclin E gene promoter partially through E2F binding sites in the promoter. In addition, the basic transcriptional factor, Sp1, might also be involved in the transactivation of the cyclin E gene by EWS-Fli1. To study the biological significance of cyclin E overexpression in EFT cells, we used flavopiridol, a pan-cyclin-dependent kinase (CDK) inhibitor and found that flavopiridol efficiently suppressed the growth of EFT cells in vitro and in vivo by the inhibition of cyclinE/CDK2 kinase activity and the induction of apoptosis. These results suggest that targeting of the cyclin/CDK complex may provide new insight into treatment of EFTs. © 2005 Wiley-Liss, Inc.