PGP and cancer: P-gp is expressed in both cancer cells and normal tissue and is involved in the absorption, distribution, and excretion of drugs (anticancer drug and so on) and toxic substances in gastrointestinal epithelial cells, proximal renal tubule epithelial cells, hepatocyte canalicular membrane, cerebral capillary endothelial cells, and placental trophoblast cells [4–6].