ABCB1 and neoplasm: On the other hand, ABCG2 is overexpressed in tumor cells, as well as other ABC transporters such as P-glycoprotein (P-gp/ABCB1) and multidrug-resistance-associated protein 1 (MRP1/ABCC1) [31], whose ability to expel chemotherapeutic agents out of the cells confers failure in antitumor therapies, along with the widely known phenomenon of multidrug resistance (MDR) [32,33,34].