CD44 and neoplasm: Several studies involving various tumor cell types have demonstrated that these giant cells are highly adaptable to hypoxic stress and acquire a mesenchymal phenotype with increased expression of CSCs markers, such as CD44, CD133, Oct4, stage-specific embryonic antigen-1 (SSEA-1), NANOG, and SOX2 [526] and ZEB1 [39,522,526,536,537,538].