NANOG and neoplasm: Numerous studies demonstrated that KLF4, SOX2, and NANOG constitute CSC markers and have key roles in sphere formation, stem cell self-renewal, cell motility, the formation and maintenance of CSC phenotype, clonogenicity and tumor regenerative ability, long-term proliferative potential of CSCs, repression of differentiation, cell cycle, migration, invasion, EMT, metastasis, and cancer progression in various malignancies [31,32].