We reasoned that the low penetrance of tumors and requirement for loss of tp53 function in our original zebrafish Ewing sarcoma model (Leacock et al., 2012) might be due to developmental toxicity of EWSR1-FLI1, similar to what was described in mouse models (Minas et al., 2017). The gene discussed is TP53; the disease is Ewing sarcoma.