ABCB1 and gastrointestinal stromal tumor: Given that decreased sensitivity of cancer cells to the multiple chemotherapeutic agents might be due to overexpression of the drug-efflux pumps that belong to the ATP-binding cassette (ABC) transporter superfamily, we examined the expression of the multidrug resistance (MDR)-associated transporter P-glycoprotein (MDR-1) or proteins from the MDR-associated protein (MRP) family in GIST T-1 vs. GIST T-1R cells.