ABCB1 (MDR1; P-glycoprotein) and ABCG2 (BCRP; MXR) are two of the most studied and well-characterized ABC drug transporters, capable of using chemical energy derived from ATP hydrolysis to transport cytotoxic anticancer drugs such as anthracyclines, paclitaxel, SN-38, topotecan, and Vinca alkaloids out of cancer cells. This evidence concerns the gene ABCB1 and cancer.