Ewing sarcoma is a malignant bone cancer that primarily occurs in children and 
adolescents. Eighty-five percent of Ewing sarcoma is characterized by the 
presence of the aberrant chimeric EWS/FLI1 fusion gene. Previously, we 
demonstrated that an interaction between EWS/FLI1 and wild-type EWS led to the 
inhibition of EWS activity and mitotic dysfunction. Although defective mitosis 
is considered to be a critical step in cancer initiation, it is unknown how 
interference with EWS contributes to Ewing sarcoma formation. Here, we 
demonstrate that EWS/FLI1- and EWS-knockdown cells display a high incidence of 
defects in the midzone, a midline structure located between segregating 
chromatids during anaphase. Defects in the midzone can lead to the failure of 
cytokinesis and can result in the induction of aneuploidy. The similarity among 
the phenotypes of EWS/FLI1- and EWS siRNA-transfected HeLa cells points to the 
inhibition of EWS as the key mechanism for the induction of midzone defects. 
Supporting this observation, the ectopic expression of EWS rescues the high 
incidence of midzone defects observed in Ewing sarcoma A673 cells. We discovered 
that EWS interacts with Aurora B kinase, and that EWS is also required for 
recruiting Aurora B to the midzone. A domain analysis revealed that the R565 in 
the RGG3 domain of EWS is essential for both Aurora B interaction and the 
recruitment of Aurora B to the midzone. Here, we propose that the impairment of 
EWS-dependent midzone formation via the recruitment of Aurora B is a potential 
mechanism of Ewing sarcoma development.