ABCG2 and neoplasm: The ATP-binding cassette (ABC) transporter proteins represent the largest family of trans-membrane proteins to confer drug resistance to tumor cells, and in particular, ABC transporter-subfamily B member 1 (ABCB1/MDR1/P-glycoprotein, P-gp), subfamily C member 1 (ABCC1/MRP1) and subfamily G member 2 (ABCG2/BCRP) are considered to be the most important transporters [64].