ABCG2 and breast cancer: Although MDR is multifactorial, it is primarily characterized by an ATP-dependent reduction in intracellular drug accumulation, due to the overexpression of three proteins belonging to the ABC transporters super family: P-glycoprotein (ABCB1), breast cancer protein (BCRP or ABCG2) or multidrug resistance-associated protein 1 (MRP1 or ABCC1).