Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Ewing sarcoma ?
    ### Response:
    What causes Ewing sarcoma? The exact cause of Ewing sarcoma remains largely unknown. Chromosomal studies have found that Ewing sarcoma cells are often characterized by an abnormal change in their genetic makeup known as a reciprocal translocation. The most common mutation, occurring in approximately 85% of Ewing sarcoma tumors, involves two genes, the EWSR1 gene on chromosome 22 and the FLI1 gene on chromosome 11.
    