Analysis of cDNA and genomlc clones shows that the murine Ig/EBP (C/EBPγ) gene encodes a small protein with a predicted molecular weight of 16.4 kDa which contains C/EBP family basic and leucine zipper domains but lacks the transcriptional activation domains present in C/EBP (CZEBPa)and NF-IL6 (C/EBPp). In transfectlon assays Ig/EBP is neither an activator nor a repressor of transcription; however, Ig/EBP inhibits the transcriptional ability of NF-IL6 (C/EBPp1) and C/EBP (C/EBPa), acting as a transdominant negative regulator. Thus Ig/EBP resembles LIP, another negative regulator of the C/EBP family, in both structure and transcriptional activity. Of the three known C/EBP family inhibitors, Ig/EBP, LIP and CHOP-10, only g/EBP is ubiquitously expressed. Therefore, Ig/EBP may act as a general buffer for C/EBP activators in many cell types.