Suggested interactions of the NANOG-OCT4-SOX2 core module of transcriptional embryonic stem cell regulation. Black, green, red and blue arrows represent complex formation, transcriptional activation, repression and translation, respectively. a: The symmetric model considered by Chickarmane et al. . b: The model suggested by Pan et al.  includes negative feedbacks through OCT4. c: The NANOG autoinhibitory circuit suggested by Navarro et al. .