In this regard, it is well established that doxorubicin is a substrate of the ATP-binding cassette (ABC) efflux transporters P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-associated proteins (MRPs), which were said to cause drug resistance in cancer therapy by reducing intracellular concentrations of doxorubicin [13,14,15,16,17,18,19,20]. Here, ABCG2 is linked to cancer.