ABCG2 and cancer: Therefore, ABCG2 has been considered as an important target in cancer treatment to improve sensitivity for chemotherapy (Breedveld et al., 2006; Hardwick et al., 2007) and ABCG2 inhibitors have been tested in clinical studies(Kruijtzer et al., 2002; Kuppens et al., 2007; Molina et al., 2008).