ABCG2 and cancer: The most significant efflux pumps found to confer chemoresistance in cancer are members of the ABC transporter family, such as ABCB1/P-glycoprotein (P-gp)/Multidrug resistance 1 (MDR1), ABCC1/multidrug resistance protein 1 (MRP1), and ABCG2/breast cancer resistance protein (BCRP) [23].