Although the strategy to interfere with the transport activity of ABCB1 (MDR1/P-glycoprotein) has not been applied successfully in a clinical setting, accumulated data obtained from both in vitro and clinical samples indicate that ABCB1 (MDR1/P-glycoprotein) serves an important role in determining phenotype, including the susceptibility of breast cancer cells to anticancer agents [5, 6, 29, 36–38]. This evidence concerns the gene ABCB1 and breast cancer.