ABCG2 and cancer: PXR regulates the expression of a number of downstream targeted genes, which are mostly related to the metabolism and transport of xenobiotics and associated with drug resistance in a number of cancers (4), such as cytochrome P450 (CYP450), multidrug resistance 1 (MDR1), breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 2 (5–7).