The study investigates a multidrug-resistant human breast cancer subline, MCF-7/AdrVp, which exhibits resistance to anthracycline anticancer drugs without overexpression of known multidrug resistance transporters like P-glycoprotein or multidrug resistance protein. RNA fingerprinting identified a 2.4-kb mRNA overexpressed in MCF-7/AdrVp cells, encoding a 663-aa member of the ATP-binding cassette (ABC) superfamily of transporters, termed breast cancer resistance protein (BCRP). Enforced expression of BCRP in MCF-7 cells confers resistance to mitoxantrone, doxorubicin, and daunorubicin, reduces daunorubicin accumulation, and enhances ATP-dependent efflux of rhodamine 123. BCRP appears to play a significant role in the multidrug resistance phenotype of MCF-7/AdrVp cells, suggesting it may be part of a multiprotein transporter complex or interact with other proteins to enhance drug resistance.The study investigates a multidrug-resistant human breast cancer subline, MCF-7/AdrVp, which exhibits resistance to anthracycline anticancer drugs without overexpression of known multidrug resistance transporters like P-glycoprotein or multidrug resistance protein. RNA fingerprinting identified a 2.4-kb mRNA overexpressed in MCF-7/AdrVp cells, encoding a 663-aa member of the ATP-binding cassette (ABC) superfamily of transporters, termed breast cancer resistance protein (BCRP). Enforced expression of BCRP in MCF-7 cells confers resistance to mitoxantrone, doxorubicin, and daunorubicin, reduces daunorubicin accumulation, and enhances ATP-dependent efflux of rhodamine 123. BCRP appears to play a significant role in the multidrug resistance phenotype of MCF-7/AdrVp cells, suggesting it may be part of a multiprotein transporter complex or interact with other proteins to enhance drug resistance.