CD44 and head and neck squamous cell carcinoma: HNSCC cells belonging to the CSC population are distinguished by their high expression of certain surface proteins (for example, CD44, CD133), pluripotency-related transcription factors (SOX2, OCT4, Nanog), and increased aldehyde dehydrogenase (ALDH) activity (Cirillo et al. 2021; Krishnamurthy et al. 2010; Prince et al. 2007).