Because NCX4040 inhibited ATPase activities of ABC transporters and was active against adriamycin-resistant ovarian tumor cells, we sought to further examine the mechanism of action of NCX4040 in detail as we believe that by understanding the mechanism of action of NCX4040, better and more active anticancer agents can be designed which may be highly effective for the treatment of human cancers in the clinic. This evidence concerns the gene ABCG2 and cancer.