Breast cancer resistance protein (BCRP1), also known as placenta-specific ATP-binding cassette (ABC) protein (ABCP) or ABC G-subfamily member 2 (ABCG2), or mitoxantrone resistance protein (MXR), is a member of the ATP-binding cassette superfamily of drug transporters thought to be involved in multi-drug resistance in human neoplastic disease. ABCG2 is expressed in drug-resistant breast, colon and gastric cancer and fibrosarcoma cancer cell lines and in blast cells from patients with acute myeloid leukemia (AML). Normal tissues that express the ABCG2 protein include placental syncytiothrophoblasts, mammary ducts and lobules, epithelium in the large intestine and colon, venous and capillary epithelium and bile canalicular membrane of the liver. Several studies indicate that ABCG2 is also expressed in stem cells of different tissues and may be useful as a stem cell marker. ABCG2 is thought to be responsible for the ability of a population of very primitive hematopoietic and non-hematopoietic cells, the so-called SP (side population) subset, to efflux the Hoechst 33342 fluorescent dye. Semiquantitative RT-PCR results indicate that ABCG2 mRNA is expressed at higher levels in SP cells as compared to non-SP cells in human, rhesus monkey and mouse hematopoietic tissues, with the exception of erythroblasts and NK cells. In human bone marrow, ABCG2 mRNA expression is higher in phenotypically defined populations of primitive CD34+CD38- hematopoietic cells as compared to more differentiated CD34+CD38+ progenitor cells. Bcrp1 mRNA is also expressed in SP cells in murine skeletal muscle, in neural stem cell and progenitor cells and in nestin-positive pancreatic islet derived progenitor cells. Over-expression of the human ABCG2 gene by retroviral-mediated gene transfer confers the SP phenotype to mouse bone marrow cells. Bcrp1 mRNA is expressed in SP as well as non-SP fractions of mouse embryonic stem cells. This suggests that Bcrp1 protein levels may not always be reflected by mRNA levels, or that other characteristics of stem cells, for example, cell cycle status or expression of other transporters, are also involved in the SP phenotype.