ABCG2 and breast cancer: In this study, we have experimentally proven that there are additional ABCG2/MDR1 drug transporter protein-expressing (SP1 and SP2) fractions in luminal breast cancer that are equally tumorigenic, such as the non-drug transporter-expressing counterparts, the MP1 and MP2 fractions, which are likely to be enriched with the luminal progenitor subpopulation.