PGP and malignant colon neoplasm: Chemosensitive human colon cancer HT29 and its chemoresistant clone HT29/MDR were incubated with the ER stress inducers thapsigargin, tunicamycin or brefeldin A, or with the chemotherapeutic agents oxaliplatin (a substrate of MRP1 and MRP4), 5-fluorouracil (a substrate of MRP1, MRP3, MRP4 and MRP5) or doxorubicin (a substrate of Pgp, MRP1, MRP2, MRP3 and BCRP), at concentrations that were cytotoxic in chemosensitive cells but not in chemoresistant ones ([7]; Additional file 1).