ABCG2 and neoplasm: Proteins of the ABC family adenosine triphosphate-binding cassette (ABC) play an important role in the development of tumor chemoresistance [9]: P-glycoprotein (P-gp), encoded by the MDR1 gene; multidrug-resistance-associated protein 1 (MRP1); breast cancer resistance protein (BRCP), encoded by the MXR gene [10, 11].