ABCG2 and hepatocellular carcinoma: MDR in HCC cells is mainly associated with two classical families of membrane transporters: (1) ATP-binding cassette (ABC) transporter superfamily, including P-glycoprotein (P-gp; also known as MDR1/ABCB1), MDR-associated protein1 (MRP1)/ABCCl, and breast cancer resistance protein (BCRP)/ABCG2.