ABCG2 and cancer: ABC transporters have broad substrate specificity in transporting the principal chemotherapeutic agents, and several non-responder cancers show a high expression of these transporters, determining a decreased therapeutic drug accumulation in tumor cells; in particular, P-gp-mediated MDR is regulated by various transcription factors and/or signaling pathways [119,120,121,122,123,124,125,126,127,128,129].