Five cDNA clones for TAXREB proteins that bind to the tax-responsive enhancer element of human T-cell leukemia virus type I (HTLV-I) were isolated from a Jurkat cell cDNA library. The beta-galactosidase fusion proteins of three of these clones specifically recognized the domain C within the enhancer. One of the three cDNAs, encoding TAXREB107, contained an open reading frame with 288 amino acid residues. RNA blot analysis showed that the level of mRNA for TAXREB107 increased transiently in Jurkat cells on treatment with TPA. Immunoblot analysis showed that polyclonal antibody against TAXREB107 specifically recognized a 34-kD protein in Jurkat cells. TAXREB107 may participate in tax-mediated trans-activation of transcription.