In 1976, Juliano and Ling(1) reported expression of a 170 kDa protein in colchicine-resistant Chinese hamster ovary (CHO) cells that was absent in drug-sensitive cells. Because this protein altered cellular permeability to colchicine, the authors named it P-glycoprotein (P-gp).(1) P-gp overexpression was described in tumor samples and leukemic cells.(2) High homology with bacterial transporters suggested that P-gp was an efflux transporter, modulating intracellular xenobiotic concentrations.(3) In 1986, the gene encoding P-gp was discovered and designated MDR1 (HUGO name ABCB1).(4) Immunohistochemical studies demonstrated P-gp expression in tissues with secretory or excretory functions (liver, kidney, and gastrointestinal tract) and at blood-tissue barrier sites, such as the blood-brain barrier.(5) This pattern of expression indicated that P-gp may influence xenobiotic response and toxicity, either through pharmacokinetic or pharmacodynamic effects.(6)