ABCG2 and cancer: We first determined the cytotoxicity of QB1561 by MTS assay in eight cancer cell lines, including the parent human epidermoid carcinoma cell line KB-3-1 and its colchicine-selected ABCB1-overexpressing cell line KB-C2, cisplatin-resistant KCP4 cells (Oiso et al., 2014), ATO-resistant ABCB6 overexpressing KB-ATO cells (Zhang et al., 2017a), human lung cancer cell line NCI-H460 and its mitoxantrone-selected ABCG2-overexpressing NCI-H460/MX20 cells, parent human colorectal cancer cell line SW620 and its adriamycin-selected ABCB1-overexpressing SW620/Ad300 cells.