ABCG2 and nasopharyngeal carcinoma: RNAi- induced depletion of endogenous Bmi-1 downregulated the mRNA levels of stem cell markers such as octamer-binding transcription factor 4 (Oct4), Nanog homeobox (Nanog), SRY-box transcription factor 2 (Sox2) and ATP binding cassette subfamily G member 2 (ABCG2) in NPC cells (Figure 4A).