RRM2 and neoplasm: The drug resistance mechanism involves metabolic abnormalities (such as downregulation of deoxycytidine kinase (dCK), nucleoside transporter hENT1 deficiency), enhanced DNA repair (overexpression of ribonucleotide reductase ribonucleotide reductase catalytic subunit M1 (RRM1)/ribonucleotide reductase catalytic subunit M2 (RRM2), and tumor microenvironment remodeling (such as secretion of immunosuppressive factors by CAFs).