We previously characterized p144 bearing N-acetylglucosamine residues in a rat liver nuclear matrix fraction. Based on partial amino acid sequences of rat p144, mouse p144 cDNA was cloned and sequenced, and its amino acid sequence was predicted. The sequence revealed that p144 is a rat homologue of CA150, which is a transcription factor involved in Tat-activated human immunodeficiency virus type 1 transcription. The reported human CA150 consists of 1098 amino acids and has a leucine zipper-like motif in its carboxyl-region. However, a clone of mouse p144 cDNA encoded a CA150 consisting of 1,034 amino acids. The mouse CA150 was shorter by 64 amino acids than hitherto known human CA150 and lacked the leucine zipper-like motif. We designated the longer and shorter CA150 species as CA150a and CA150b, respectively. The partial nucleotide sequences of other mouse p144 cDNA clones were examined and it was found that some clones encode CA150a having a leucine zipper-like motif. It was suggested that CA150a and CA150b are splicing isoforms. All rat and mouse tissues examined contained transcripts for both CA150a and CA150b. Both transcripts were detected in human blood and Jurkat cells as well as mouse CD4(+) T-cells, which are the HIV-1-sensitive counterpart in humans.