ABCG2 and neoplasm: The overexpression of ATP binding cassette (ABC) transporters - such as P-glycoprotein (Pgp/ABCB1), multidrug resistance related proteins (MRPs/ABCCs) and breast cancer resistance protein (BCRP/ABCG2) - limits the intracellular retention and activity of several cytotoxic drugs, producing a multidrug resistant (MDR) phenotype in tumor cells [6].