EWSR1 and Ewing sarcoma: To further confirm SPOP as a physiological E3 ligase for EWS–FLI1, we depleted endogenous SPOP in 4 Ewing sarcoma cell lines (A673: Figure 1H and Figure S1I (Supporting Information); MHH‐ES‐1: Figure 1I; SK‐N‐MC: Figure S1J (Supporting Information); and EWS894: Figure S1K (Supporting Information)).