NANOG and cancer: The status of stem cell markers is quantified through tumor spheroid formation and the expression of select molecules, including clusters of differentiation clusters of differentiation CD44 and CD133, nuclear transcription factors octamer-binding transcription factor-4 (OCT-4), sex determining region box Y-2 (Sox-2), Kruppel-like factor-4 (Klf-4), cellular Myc (c-Myc), and DNA-binding transcription factor NANOG in cancer stem cell models [87,88,89].