ABCG2 and osteosarcoma: In fact, osteosarcoma stem cells have been shown to overexpress self-renewal and pluripotency markers, such as OCT4, SOX2 and NANOG, and drug transporters, such as the Mitoxantrone resistance protein (MXR/BCRP1/ABCG2), which has been shown to participate in the multi-drug resistance of osteosarcoma[31–33].