ABCG2 and neoplasm: Another CSC marker in the form of overexpressed ABCG2 has been linked with the regulation of hypoxia-induced chemoresistance to gemcitabine [28], moreover, ABCG2 expression induced by gastrin has been reported to elevate the proportion of SP cells, increasing the possibility of tumor cell metastasis potential and activity of cell invasion by activating NF-κB signaling in PC [29].