To identify cellular factors which associate with HIV-1 activation, we have isolated 7 clones with their TAR RNA binding properties. Data base search revealed that 5 clones were identified as a nuclear factor 90 (NF90), one of poly A binding proteins, a human homologue of yeast 35.1kDa protein, a human homologoue of chicken inner centromere protein, and a human homologue of mouse phosphoprotein p150, respectively. The remaining 2 clones have not been identified. NF90, which is indispensable for activated T-cells to produce interleukin-2, has two domains, which contains a double-stranded RNA binding motif common to such as TAR RNA-binding protein (TRBP). TRBP is the first identified gene encoding a TAR RNA-binding protein. Thus, our screening conditions are seemed to be suitable for screening of TAR RNA-binding proteins from cDNA libraries constructed by the lambdagt11 vector. As for clone 27 (JTR27 ; a human homologue of yeast 35.1kDa protein), its full-length cDNA was sequenced and its characters were investigated. JTR27 antisense oligonucleotide repressed HIV-1 LTR-directed gene expression either in the presence or absence of Tat. Overexpression of JTR27 significantly augmented not only the HIV-1 LTR basal transcription but also Tat-activation. Taken together these results, it was suggested that JTR27 associates with HIV-1LTR gene expression.