ABCG2 and neoplasm: Chemotherapy resistance in GC is also associated with drug efflux caused by the overexpression of some membrane transporters, the most important of which is the ATP-binding cassette (ABC) transporter family (30, 31), which is represented by P-glycoprotein (P-gp), which can pump anti-tumor drugs from inside to outside so that tumor cells can escape from the cytotoxic effect of and show resistance to chemotherapy drugs (30).