NANOG core circuit models studied in this work. We consider the OCT4/SOX2 dimer as a common transcription factor for all three genes, and the NANOG protein to be a transcription enhancer for the Sox2 gene. We investigate model scenarios in which NANOG is (a) or is not (c) an inducer of Oct4. Furthermore, we consider further variants, such as the model proposed by Pan et al. where high OCT4 levels repress Nanog (b), as well as the NANOG autorepressor feedback proposed by (d) and a combination of NANOG autorepressor feedback with OCT4 repression (e).