TREB5/hXBP-1/HTF is a basic region leucine zipper protein which binds to a cyclic AMP responsive element (CRE)-like element in both human T-cell leukemia virus type 1 and human major histocompatibility complex (MHC) class II genes. To analyze the structure and transcription regulation of the TREB5 gene, we isolated the mouse TREB5 gene and cDNA. The mouse TREB5 gene contains five exons and four introns and spans approximately 5 kb. The deduced amino acid sequence of mouse TREB5 exhibited 77% and 94% homology to human and rat TREB5, respectively. The b-zip structure is completely conserved in mouse, rat and human. Southern blot analysis of the mouse genomic DNA demonstrated that positive bands exactly coincide with those expected from sequences of the cloned genes, indicating that the mouse TREB5 gene is present as a single copy. The transcription start site of the mouse TREB5 gene was mapped to -15 bp upstream from the ATG initiation codon. Promoter analysis of serial deletion mutants revealed that the -142 bp upstream region is the minimum sequence to promote mouse TREB5 gene expression and that the -1.0 kb upstream region is required for full promoter activity.