ABCB1 and neoplasm: Multiple drug resistance (MDR) in tumor cells is often associated with the overexpression of ATP-binding cassette (ABC) transporter proteins, including P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1), multidrug resistance-associated protein 6 (MRP6) and lung resistance-associated protein (LRP), which mediate tumor cell resistance to chemotherapeutic drugs and inhibit tumor cell killing by reducing the concentration of chemotherapeutic drugs in the cells [26–30].