ABCB1 and cancer: Drug resistance and nonselective toxicity have been two major clinical obstacles for cancer chemotherapy.1, 2, 3, 4, 5 Drug resistance is responsible for treatment failure in >90% of patients with metastatic cancers.6 A major cause of drug resistance is the overexpression of a 170-kDa transmembrane protein, P-glycoprotein (P-gp), on the cancer cell surface in response to drug treatment.