CCAAT-enhancer binding protein epsilon (CEBP epsilon) is a 32 kDa basic-leucine zipper transcription factor that is expressed in myeloblasts, granulocytes, and eosinophils. Multiple isoforms of human CEBP epsilon are generated by alternate splicing and alternative translation initiation sites. All four isoforms contain the bZIP domain (aa 208-267) but exert different effects on myeloid differentiation. Full length CEBP epsilon and the 30 kDa isoform (aa 33-281) cooperate with c-Myb to activate the transcription of genes involved in myeloid differentiation. The 27 kDa isoform (which lacks aa 69-97) and the 14 kDa isoform (which begins at Met153) function as transcriptional repressors in eosinophils.