Resistance to doxorubicin in MCF-7/DOX cells. (A, B) Cell proliferation in MDA-MB-231, MCF-7, MCF-7/DOX, and T-47D breast cancer cells 72 h after treatment with doxorubicin at (A) high or (B) low concentration. (C) The number of apoptotic cells was counted by terminal deoxynucleotidyl transferase dUTP nick end labeling-based fluorescence-activated cell sorting analysis 48 h after doxorubicin treatment. (D) Western blot analysis of MCF-7 and MCF-7/DOX cells following treatment with doxorubicin for 48 h. Values shown are means ± standard deviation (SD). *P < 0.01; **P < 0.001 compared with the untreated group.