Key proteins from the ATP-binding cassette (ABC) transporter family, such as P-glycoprotein (P-gp or multidrug resistance protein 1 (MDR1)/ATP-binding cassette sub-family B member 1 (ABCB1)), breast cancer resistance protein (BCRP or ATP-binding cassette sub-family G member 2 (ABCG2)), and major vault protein (MVP) or lung resistance-related protein (LRP), contribute to chemoresistance by facilitating the efflux of drugs from cancer cells [16,17,18,19,20]. The gene discussed is ABCG2; the disease is cancer.