ABCG2 and cancer: P-glycoprotein (P-gp; ABCB1) and ABCG2 (BCRP; MXR) are members of the ATP-binding cassette (ABC) drug transporter family that are characteristically known for utilizing energy derived from ATP hydrolysis to actively efflux a wide range of cytotoxic and molecularly targeted anticancer agents out of cancer cells [6,7,8,9,10].