Initial attempts to generate tumors with RSG-EWSR1-FLI1 driven by the beta-actin promoter (Figure 1—figure supplement 1B and Figure 1—figure supplement 2B, C) caused early apoptosis (Figure 1—figure supplement 2A) and the growth of cranial cell masses (Figure 1—figure supplement 2D), however failed to recapitulate the histologic appearance of Ewing sarcoma (Figure 1—figure supplement 2E). The gene discussed is ACTB; the disease is Ewing sarcoma.