High expressions of ABCG2, ABCB1, or ABCC1 were validated in CSCs in various cancers, and the ability of CSCs to extrude Hochest 33342 by ABCG2 was used to develop the “side population” method, which is frequently used to isolate stem-like cells from primary tumors or cancer cell lines [102]. This evidence concerns the gene ABCG2 and cancer.