ABCG2 and head and neck squamous cell carcinoma: To confirm the role of ABCG2 in SP Hoechst 33342 efflux phenotype and chemoresistance in HNSCC, we over-expressed ABCG2 in non-SP cells and found that the over-expression of ABCG2 could promote non-SP cells to acquire SP Hoechst 33342 efflux phenotype and chemoresistance.