CCAAT-enhancer binding protein alpha (C/EBP alpha) is a widely expressed 42 kDa basic-leucine zipper transcription factor that forms functional homodimers and heterodimers with C/EBP beta and gamma. C/EBP alpha inhibits mitotic progression through multiple mechanisms and also promotes terminal differentiation of granulocytes and adipocytes. Its activity is regulated by SUMOylation and by phosphorylation at multiple serine residues. C/EBP alpha contains three TE transactivation domains (aa 1-69, aa 70-96, and aa 126-200) and one basic DNA binding motif and leucine zipper (aa 286-345). A 30 kDa isoform is generated by the use of an internal translation initiation site. The p30 isoform functions as a dominant negative factor by heterodimerizing with and inhibiting full length C/EBP alpha activity. Within aa 1-124, human C/EBP alpha shares 93% amino acid sequence identity with mouse and rat C/EBP alpha, respectively.