An ABC transporter. Allows efflux of Hoechst dye, a property that has been used to separate bone marrow side population cells, which express BCRP/ABCG2. Appears to play a major role in the multidrug resistance phenotype of a specific MCF-7 breast cancer cell line. When overexpressed, the transfected cells become resistant to mitoxantrone, daunorubicin and doxorubicin, display diminished intracellular accumulation of daunorubicin, and manifest an ATP-dependent increase in the efflux of rhodami