ABCG2 and cholangiocarcinoma: Currently, 49 ABC transporter subtypes have been identified; of these, ABC subfamily G, isoform 2 protein (ABCG2), and multidrug resistance protein 1 (MRP1) have been implicated in the resistance of cholangiocarcinoma cells to chemotherapeutic agents (such as 5-fluorouracil, taxane derivatives, and doxorubicin) [14 ,15].