ABCG2 and breast cancer: ABCG2, also named mitoxantrone resistance-associated protein (MXR), breast cancer resistance protein (BCRP) or placenta-specific ATP-binding cassette transporter (ABCP), was first cloned from drug selected human breast cancer cells MCF-7 and has been reported to transport chemotherapeutic agents including rhodamine 123, doxorubicin, mitoxantrone, topotecan and imatinib [2], [7]–[11].