A novel human cDNA containing CAG repeats, designated B120, was cloned by PCR amplification. An approximately 300-bp 3' untranslated region in this cDNA was followed by a 3426-bp coding region containing the CAG repeats. A computer search failed to find any significant homology between this cDNA and previously reported genes. The number of CAG trinucleotide repeats appeared to vary from seven to 12 in analyses of genomic DNA from healthy volunteers. An approximately 8-kb band was detected in brain, skeletal muscle and thymus by Northern blot analysis. The deduced amino-acid sequence had a polyglutamine chain encoded by CAG repeats as well as glutamine- and tyrosine-rich repeats, which has also been reported for several RNA binding proteins. We immunized mice with recombinant gene product and established a monoclonal antibody to it. On Western immunoblotting, this antibody detected an approximately 120-kDa protein in human brain tissue. In addition, immunohistochemical staining showed that the cytoplasm of neural cells was stained with this antibody. These findings indicated that B120 is a novel cDNA with a CAG repeat length polymorphism and that its gene product is a cytoplasmic protein with a molecular mass of 120 kDa.