AbstractObjectiveEWS‐FLI1 is the most common oncogenic fusion protein in Ewing's sarcoma family tumors (ESFTs). DAX1, an orphan member of the nuclear receptor superfamily, is up‐regulated by EWS‐FLI1 and plays a key role in the transformed phenotype of ESFTs.MethodsTo discover a functional inhibitor of DAX1 and EWS‐FLI1, we screened small‐molecular inhibitors using a DAX1 reporter assay system.ResultsK‐234 and its derivatives, which were dihydroorotate dehydrogenase (DHODH) inhibitors, showed inhibitory effects in the reporter assay. K‐234 inhibited the growth of Ewing's sarcoma with various fusion types, and K‐234 derivatives altered the expression of EWS‐FLI1‐regulated genes. The DAX1 expression had no effect on the growth inhibitory effect of the K‐234 derivatives, while DHODH overexpression or uridine treatment attenuated their inhibitory effects, suggesting that inhibition by K‐234 derivatives occurs through DHODH inhibition. An in vivo study showed that a K‐234 derivative clearly inhibited tumor growth in an Ewing's sarcoma xenograft mouse model.ConclusionTaken together, the present results suggest that DHODH inhibitors can inhibit the function of DAX1/EWS‐FLI1 in ESFTs and might be a therapeutic agent with potent anti‐tumor activity for Ewing's sarcoma patients.