ABCG2 and breast cancer: Similarly, it has been reported previously that methylxanthine blocks the nuclear enzyme poly (ADP-ribose)polymerase1 (Geraets et al., 2006), rendering tumor cells more sensitive to chemotherapeutic drugs (Rosendahl et al., 2015) and reverses breast cancer cell resistance to various conventional drugs by downregulating the breast cancer resistance protein (BCRP/ABCG2) (Ding et al., 2012).