PGP and breast cancer: Although one study suggests P-gp is also located in mitochondria in doxorubicin-resistant K562 human leukemia cells (15), others reveal that P-gp is not presented in mitochondria either in MCF-7 (ADR) human breast cancer and KB-V1 human cervix carcinoma drug resistant cell lines (16) or in primary rat hepatocytes (17).