ALDH1A1 and head and neck squamous cell carcinoma: Among the 28 CSC genes encoding CSC biomarker proteins that have been validated in HNSCC [10], we selected seven CSC genes that satisfied the following criteria, CD44, MET, ALDH1A1, BMI1, PROM1, SOX2, and POU5F1 from a literature search: (a) showing clinical significance associated with the expression of corresponding CSC biomarker proteins in HNSCC and (b) studied more than twice over the past 10 years [5, 8, 9, 22–30].