ABCG2 and osteosarcoma: Four members of ABC family have been shown to contribute to multidrug resistance in osteosarcoma, including MDR1 (also called P-glycoprotein, P-gp or ABCB1), multidrug resistance-associated protein 1 and 2 (MRP1/ABCC1 and MRP2/ABCC2), and breast cancer resistance protein (BCRP/ABCG2) [5].