NQO1 and breast cancer: We compared two breast-cancer cell lines, MCF7 and MDA-MB-231, which are known as negative and positive cell lines for BCSC markers, respectively [47] Quantitative RT-PCR indicated that the messenger RNA (mRNA) expression level of NQO1 is very low in BCSC-marker-positive cells (MDA-MB-231) compared to BCSC-negative cells (MCF-7) (Figure 1A).