RRM1 and cancer: The most common cancer-associated biomarkers identified by IHC were the multidrug resistance-associated protein 1 (MRP1) (89%; 8/9 tumors), tubulin beta 3 class III (TUBB3) (86%; 18/21 tumors), phosphatase and tensin homolog (PTEN) (85%; 28/33 tumors), topoisomerase 2A (TOP2A) (84%; 26/31 tumors), thymidylate synthase (TS) (80%; 24/31 tumors); ribonucleotide reductase M1 (RRM1) (71%; 15/21 tumors), and topoisomerase 1 (TOP1) (63%; 19/30 tumors) (Fig. 1).