ABCG2 and neoplasm: The abnormal expression of ABC (ATP-binding cassette) [5,6,7] transporters is a significant cause of MDR produced by cancer cells, including P-glycoprotein (P-gp) [8,9,10], breast cancer drug resistance protein (BCRP) [11], multidrug resistance-associated protein (MRP) [12], etc. Among them, P-glycoprotein (P-gp) is the most representative member of the ABC transporter family, which is encoded by the MDR1 gene [13] and overexpressed in a variety of tumor-resistant strains.