ABCG2 and nasopharyngeal carcinoma: ABCG2, a member of ABC transporter superfamily, is a transmembrane protein in charge of the efflux of chemotherapy drugs, metabolites and other compounds such as Hoechst 33342 dye, thus it is responsible for the phenotype of SP cells and has been widely used as a marker in CSCs isolated from retinoblastoma (45), embryonic (44), lung (41), liver (46), pancreas (47), gallbladder (48) and head and neck cancers (55) and NPC (23).