Tat binding protein-1 (TBP-1) is one of the molecules that interact with HIV Tat protein and have influence on Tat-mediated transactivation of HIV. In addition, TBP-1 has been recognized as a component of the 19S regulatory subunit of the multiprotein complex, the 26S proteasome, that is essential for many basic events in cells, e.g. cell cycle regulation, by degrading the ubiquitinized proteins. Here we have cloned a mouse TBP-1, confirmed its inhibitory action on Tat activity and revealed its in vivo heterogeneous expression pattern in the mouse. The cloned mouse TBP-1 cDNA is 1569 bp in size, longer than reported human TBP-1 cDNA, and another possible initiation site has been identified. Robust expression of TBP-1 mRNA can be observed in the testis, especially in the spermatogonia and spermatocytes. Our immunohistochemical study has revealed that TBP-1 is mainly localized in the nuclei of these testicular cells. Expression of TBP-1 mRNA in CD4+ lymphocytes is confirmed by RT-PCR. Localization of TBP-1 transcripts in vivo is similar to the reported distribution of constitutive components of the 20S proteasome. The heterogeneous and restricted expression of TBP-1 in vivo suggests that there may be a diverse reaction of tissues against the HIV proliferation and a diverse capability of tissues in degrading ubiquitinized proteins in vivo.