ABCG2 and head and neck squamous cell carcinoma: Several markers have been proposed to identify CSCs in HNSCC, including CD133, CD44, ATP-binding cassette sub-family G member 2 (ABCG2), stemness-related transcription factors Nanog, Octamer binding transcription factor 4 (Oct4), sex determining region Y (SRY)- box 2 (Sox2), and aldehyde dehydrogenase (ALDH) activity [6, 10–14].