ABCG2 and cancer: Moreover, mortalin overexpression causes an increase in cancer cell stemness by enhancing the expression levels of stem cell markers, such as ATP binding cassette subfamily G member 2 (ABCG2), POU class 5 homeobox 1 (POU5F1/OCT-4), CD133, aldehyde dehydrogenase 1 (ALDH1), CD9, ATP binding cassette subfamily C member 1 (ABCC1/MRP1), and connexin (Yun et al., 2017).