CSCs also impact multidrug resistance in BC through the overexpression of ATP-binding cassette (ABC) transporter proteins, ABCB1 (P-glycoprotein), ABCC1 and ABCG2 (BCRP1), which reduces the concentrations of chemotherapeutic drugs to suboptimal levels in the TME [93,94]. This evidence concerns the gene ABCC1 and breast cancer.