Ewing sarcoma cells proliferate optimally at specific levels or activity of EWSR1::FLI1 but acquire mesenchymal features and eventually die when this oncogene is either silenced (EWSR1::FLI1 KD) or overexpressed (EWSR1::FLI1 OE)3–7. This evidence concerns the gene FLI1 and Ewing sarcoma.