Cancerous cells might also express a number of these membrane pumps (e.g., P-glycoprotein (Pgp, MDR1, ABCB1), multidrug-resistance protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2)) since many chemotherapy drugs are substrates of the above-mentioned transporters and, thereby, drugs can be expelled from cancer cells. This evidence concerns the gene ABCG2 and cancer.