c-Myc (MYC) is a helix-loop-helix leucine zipper transcription factor that dimerizes with its partner protein Max to bind specific DNA sequences and transactivates genes. The Myc-Max heterodimer can also repress gene expression through complex formation with the transcription factor Miz1 [PubMed: 18923074]. In addition to its role in cancer, Myc is one of four transcription factors that collectively can re-program differentiated adult cells back to a pluripotent stem cell state [PubMed: 16904174]. Myc also plays an important role in normal cell physiology. The key distinction between physiological and oncogenic Myc function is whether MYC expression is regulated by normal circuitries, such as growth factor signaling that occurs when cells enter into the cell cycle and proliferate for tissue repair or whether, as in cancers, MYC activation can be short-circuited by genetic alterations, permitting deregulated Myc expression to alter transcription that no longer responds to external cues, particularly negative regulatory ones [PubMed: 16267388].