The transcription factor Yin Yang 1 (YY1) (also known as NF-E1, δ, CP-1 and UCRBP) is a GLI-Kruppel zinc finger protein. 1–4YY1 is highly conserved from Xenopus to mammalian YY1 and has been demonstrated to play an essential role in mouse embryonic development and other physiological and pathological conditions.5, 6 As one of the first mammalian transcription factors that have been identified to possess both repression and activation activities, YY1 has served as a model for understanding how a single molecule can conduct opposite transcriptional functions, that is, repression and activation. Recent studies suggest that YY1 is a mammalian counterpart of the Drosophila Polycomb Group (PcG) protein Pleiohomeotic (Pho),7, 8 providing a new framework in which to further explore YY1’s biological functions and mechanism of action in vivo. In this chapter, we discuss progress on YY1 since its discovery.