It has long been postulated that the multidrug efflux transporter P-glycoprotein (P-gp/ABCB1/MDR1) mediates the main mechanism of resistance of cancer cells to chemotherapeutic agents; however, recent studies have shown that the Hh receptor Ptch1, which is over-expressed in many recurrent and metastatic cancers, pumps chemotherapeutic agents such as doxorubicin out of cancer cells, and thereby also contributes to chemotherapy resistance. This evidence concerns the gene ABCB1 and cancer.