ABCG2 and cancer: Among the ATP-binding cassette (ABC) transporter superfamily, multidrug resistance proteins (MRPs), breast cancer resistance protein (BCRP/ATP Binding Cassette Subfamily G Member 2 (ABCG2)), and P-glycoprotein (P-gp/MDR1/ATP Binding Cassette Subfamily B Member 1 (ABCB1)), as ATP-driven efflux pumps, characterize classic MDR in cancer cells [3].