ABCG2 and neoplasm: In addition to facilitating drug efflux by exosomes, drug exporter pumps, such as P-glycoprotein (P-gp/MDR1), multidrug-resistant protein-1 (MRP-1), and breast cancer resistance protein (BCRP/ABCG2) participate in drug efflux from the cells, therefore overexpression of these pumps in tumor cells can decrease the drug bioavailability and establish drug resistance [35, 36].