CD44 and head and neck squamous cell carcinoma: In HNSCC, several putative CSC markers, such as CD44 [20], CD133 [21], CD98 [22], CD10 [23], side population (SP) [24], aldehyde dehydrogenase (ALDH) activity [25] and ZsGreen-cODC (C-terminal sequence that directs degradation of ornithine decarboxylase) [26] were described as indicators of tumor cell populations with enhanced tumorigenic potential and reduced sensitivity to chemo- or radiotherapy.