The EWS-FLI-1 gene product regulates the expression of a number of genes important for cancer progression [7], can transform mouse cells such as NIH3T3 [8] and C3H10T1/2 [9], and is necessary for proliferation and tumorigenicity of Ewing sarcoma cells [1-5], suggesting that EWS-FLI-1 is the causative oncogene. This evidence concerns the gene EWSR1 and Ewing sarcoma.