To understand immune contexture–dependent contributions, we conducted parallel analyses of identical Ewing tumors in immunodeficient NOD/SCID gamma (NSG) mice and found that Ewing sarcoma tumors developed in an immunocompetent, humanized murine model have increased expression of important modulators of the TME, including TGFB1, and increased metastatic potential, as compared with tumors developed in NSG, immunodeficient models. Here, TGFB1 is linked to Ewing sarcoma.