ABCB1 and neoplasm: MDR in CRC appears to be mediated by a series of mechanisms which could be divided into two principal types of resistance: “pump” and “nonpump.” The main mechanism of pump resistance is the increased ability of tumor cells to efflux drugs, which is induced by the ATP-binding cassette (ABC) superfamily of membrane transporters, including pump P-glycoprotein (P-gp), breast cancer resistance-associated protein (BCRP), and multidrug resistance-associated protein (MRP) subfamily.