ABCG2 and cancer: The most simple mechanism of the development of multidrug resistance is as follows: anti-tumor drug molecules flow out from cancer cells through membrane channel proteins driven by ATP, especially by adjustment of P-glycoprotein pump (Pgp-pump) and due to the effects of breast cancer resistance protein-1 (BCRP/ABCG-2) and multidrug-resistance-associated protein-1 (MRP-1) from thymic cancer cells [52].