ALDH1A1 and head and neck squamous cell carcinoma: Accumulating data have confirmed that ALDH1A1 not only stands out as a CSC biomarker but is also associated with tumor metastasis and poor prognosis in different tumor types, such as breast cancer,9 PCa,14 HNSCC,13 bladder cancer,16 GBM,112 clear cell renal cell carcinoma (ccRCC),113 and GC.114