ABCG2 and cancer: The upregulation of ATP-binding cassette (ABC) transporters, such as ABCB1 (P-glycoprotein), ABCG2 (breast cancer resistance protein), and ABCC1 (multidrug resistance-associated protein), facilitates the efflux of 5-FU from cancer cells, diminishing its intracellular concentration and efficacy [18,19].