ABCG2 and cancer: Cancer patients who are resistant to chemotherapy often exhibit high expression of various ABC transporter efflux pumps including, multi-drug resistance protein 1 (P-gp/MDR1), MDR-associated protein 1 and breast cancer resistance protein (BCRP) (17, 18), the ABC transporter family members requiring ATP hydrolysis to efflux substrates and cytotoxic substances from cells (19).