EWSR1 and Ewing sarcoma: Although EWSR1::FLI1-bound GGAA repeats are devoid of H3K27me3 both in Ewing sarcoma cell lines and primary tumors (16, 17), in human umbilical vein endothelial cells and hpMSC, these regions are extensively decorated with H3K27me3 (8, 39).