ABCG2 and breast cancer: To investigate the effects of ABCG2-siRNA in reversing MDR in mice bearing Adriamycin-resistant human breast cancer cells (MCF-7/ADR) xenograft tumors, using Adriamycin-sensitive human breast cancer cells (MCF-7/S) xenograft tumors as a control, we prepared siRNA-loaded PEAL NPs by the emulsion-solvent evaporation method for systemic siRNA delivery (Supplementary Scheme S1) [26].