RRM1 and Ewing sarcoma: However, before knocking out the RRM1 gene, we validated that the endogenous RRM1 protein is expressed at a stable level throughout the cell cycle in Ewing sarcoma cells by labeling EW8 cells with a fluorescent, cell-cycle reporter (PIP-FUCCI) and using flow cytometry to isolate cells in G1, S, and G2–M phases of the cell cycle (27).