SOX2 and head and neck squamous cell carcinoma: Bmi-1 knockdown suppresses the expression of stemness genes, including Oct-4, Nanog, Sox2, Nestin, as well as c-Myc and β-catenin in HNSCC ALDH1+ cells, and leads to a reduction in foci formation, migration, invasion, and tumour sphere formation [8,11,18,21,28,39,41], together with a downregulation of the drug-resistant genes MDR-1, MRP-1, and ABCG2 [12] (Table S4).