## Chromosomal Translocation in Ewing Sarcoma **Key Point:** The t(11;22)(q24;q12) translocation resulting in EWSR1-FLI1 fusion gene is present in ~90% of Ewing sarcomas and is considered pathognomonic for the disease. **High-Yield:** This translocation is detected by: - Fluorescence in situ hybridization (FISH) - Reverse transcription polymerase chain reaction (RT-PCR) - Cytogenetics ### Mechanism The EWSR1 gene (Ewing sarcoma breakpoint region 1) on chromosome 22 fuses with the FLI1 transcription factor gene on chromosome 11. This creates an aberrant fusion protein that acts as a potent oncogenic transcription factor, driving uncontrolled cell proliferation. ### Alternative Translocations Rarer variants (~10%) include: - t(21;22) — EWSR1-ERG - t(7;22) — EWSR1-ETV1 - t(17;22) — EWSR1-E1AF All involve EWSR1 on chromosome 22 paired with different ETS family transcription factors. **Clinical Pearl:** Detection of EWSR1-FLI1 is not only diagnostic but also prognostic — the type of fusion (FLI1 vs. ERG) may influence treatment response and outcome. 
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