The CRES protein is a member of the cystatin superfamily of cysteine protease inhibitors with restricted expression in stage-specific germ cells, proximal caput epididymidis, and anterior pituitary gonadotroph cells. To elucidate the molecular mechanisms regulating the highly restricted expression of the cres gene, we have sequenced 1.6 kilobases of mouse cres 5′ flanking sequence and performed studies to examine the cres gene promoter. Two putative CCAAT/enhancer binding protein (C/EBP) transcription factor binding motifs exist within the first 135 base pairs of cres promoter. Furthermore, our studies demonstrate that cres mRNA levels are dramatically reduced in the epididymides of C/EBPβ-deficient mice. These data suggest that the C/EBP family of transcription factors, in particular C/EBPβ, plays a role in the regulation of cres gene expression. In support of this finding, Northern blot analysis showed that C/EBPβ is the predominant C/EBP family member expressed in the LβT2 gonadotroph cell line and the proximal caput epididymidis. Also, gel shift and supershift assays demonstrated that C/EBPβ protein in nuclear extracts from LβT2 gonadotroph cells and epididymal cells bound to the two C/EBP sites in the cres promoter. Finally, to test the in vivo function of the C/EBP sites in cres gene expression, transfection studies were performed in LβT2 gonadotroph cells and two heterologous cell systems. These experiments showed a significant reduction of cres transactivation when either C/EBP sites were mutated, and no transC/EBP activation of the cres promoter when both C/EBP sites were mutated. Taken together, these studies demonstrate that the C/EBPβ transcription factor is necessary for high levels of cres gene expression in the proximal caput epididymidis and anterior pituitary gonadotroph cells.