ATP-binding cassette (ABC) transport proteins, such as P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP), can transport chemotherapy drugs, including 5-FU, out of tumor cells to decrease the cytotoxicity of drug 18,19. The gene discussed is ABCG2; the disease is neoplasm.