CDK4 and Ewing sarcoma: We previously established a patient-derived orthotopic xenograft (PDOX) models of a rare case of ES with both a FUG-ERG fusion [10, 11] and a loss of the CDKN2A. Previously, we reported that a CDK4/6 inhibitor and insulin-like growth factor-1 receptor (IGF-1R) inhibitor were effective in the ES PDOX model [12].