EWSR1 and Ewing sarcoma: Perturbation of alternative splicing programs is a feature of Ewing sarcoma, and has been attributed to the presence of oncogenic fusion proteins (e.g. EWS-FLI1) (10–12) However, while EWS-FLI1 function in splicing was thought to be due to its EWS moiety, our findings demonstrate that the FLI1-derived moiety is also a major contributor.