ABCG2 and ovarian carcinoma: A commonly proposed mechanism of multi-drug resistance (MDR) in recurrent ovarian cancer is elevated drug efflux, which is often due to increased activity of the ATP-binding cassette (ABC) transporter ABCB1 (previously named MDR1) gene that encodes P-glycoprotein (P-gp)[4] A2780ADR cells, previously developed by culturing parental A2780 cells in the presence of doxorubicin, have gained resistance to the drug by overexpressing P-gp, [22] which we confirmed by flow cytometry with a fluorescence-tagged antibody to P-gp (Figure 4C, left).