EWSR1-FLI1 is a fusion protein that results from a genetic alteration involving the EWSR1 gene and the FLI1 gene. This fusion is commonly associated with certain types of cancer, particularly Ewing sarcoma, a rare and aggressive bone cancer that primarily affects children and young adults. The fusion protein plays a crucial role in the development of tumors by influencing cell growth and differentiation. The presence of the EWSR1-FLI1 fusion can be detected through various diagnostic methods, including genetic testing and molecular analysis. Understanding this fusion is important for developing targeted therapies and improving treatment outcomes for patients diagnosed with Ewing sarcoma.