RRM2 and cancer: Biomarkers are highly needed for prediction of responses to these inhibitors in cancer patients and several biomarkers have been suggested, such as p53 mutation, replication stress associated markers (γH2AX, oncogene expression and low levels of ribonucleotide reductase subunit RRM2) [3,22,45,46], and multiple other regulators of DNA damage and cell cycle, such as ATM, CDC25A, MYT1, p21, or ATR signaling [27,47,48,49,50].