CDKN2A and Ewing sarcoma: However, a variety of evidence also suggest that EWS-FLI-1 alone cannot fully explain the Ewing sarcomagenesis: 1) EWS-FLI-1 alone cannot transform any human cell types including mesenchymal stem cells (MSCs) which are the putative cells of origin of Ewing sarcoma [1-4]; 2) Generating a transgenic mouse model of Ewing sarcoma by using EWS-FLI-1 alone has been unsuccessful [1-3]; and 3) Other genetic alterations such as mutations of INK4a and p53, although far less common than EWS-FLI-1 translocation, confer worse clinical outcome [1, 2].