PGP and cancer: But the development of MDR after a period of chemotherapy often made them useless.[46] Over-expression of ABC transporters has been shown to be responsible for MDR.[47] The most typical ABC transporters in the cell membrane was P-gp.[48,49] P-gp was able to limit the effectiveness of chemotherapy in a variety of common malignancies and was responsible for overall poor efficacy of cancer chemotherapy.[50,51] Therefore, reversing of multidrug resistance is thought to be a useful way to enhance the effects of chemotherapy.