ABCG2 and neoplasm: As the primary active drug efflux proteins, the ATP-binding cassette (ABC) transporter family, including P-glycoprotein (P-gp), multidrug resistance-associated protein (MRP) and breast cancer resistance protein (BCRP), utilizes the energy provided by ATP to pump drugs out of tumor cells to protect tumor tissue from chemical toxicity, leading to an insufficient intracellular concentration and a poor therapeutic efficiency of drugs [10, 11].