Nevertheless, the best responses to chemotherapy are seen in functionally negative tumors, i.e., in those tumors which do not present ABC-transporters capable of extruding compounds from the intracellular compartment, while the worst responses are more often registered in functionally P-gp/BCRP–positive tumors that bear ABC-transporters which pump anti-cancer drugs out of the cell preventing their action. This evidence concerns the gene ABCG2 and cancer.