DHFR and cancer: Amplification of drug resistance genes, including dihydrofolate reductase (DHFR) (35, 36), Multidrug Resistance Protein 1 (MRP1 or ABCB1) (36), Multidrug Resistance-Associated Protein 1 (MRP1 or ABCC1) (37, 38), Thymidylate Synthase (TYMS) (21), and Human Epidermal Growth Factor Receptor 2 (HER2/ERBB2) (2), have been identified as mediators of chemoresistance in cancer.