RRM2 and breast cancer: The ribonucleotide reductase M2 subunit (RRM2) plays important role in several human cancers, including colorectal cancer, hepatocellular carcinoma, pancreatic adenocarcinoma, and breast cancer (Lee et al., 2014; Sierzega et al., 2017; Nana et al., 2018; Chen et al., 2019).