ABCB1 and cancer: Two cell types were chiefly used [203]: (1) SKVLB1 cells, which over-express P-glycoprotein and hence pump-out xenobiotics from the intracellular space, thereby thwarting the action of anti-cancer drugs; and (2) SKOV3 cells, which lack the over-expressed P-glycoproteins and hence are drug-sensitive (thereby providing a negative control).