Abstract The human DNA- and RNA-binding protein JKTBP is a new member of heterogeneous nuclear ribonucleoproteins (hnRNPs) that are involved in mRNA biogenesis. We cloned and characterized a mouse homolog and studied its expression in mouse tissues. The cDNA encoded a 301-residue polypeptide. There is only a single amino acid difference between the mouse and human sequences. Northern blotting indicated ubiquitous but varied expressions of approximately 1.4 and 2.8 kb mRNAs in various tissues. Immunoblotting indicated that the amounts of protein of about 38 kDa were higher in the brain and testis than in other tissues. An additional protein of about 53 kDa was found in the brain and testis. Germ cell-deficient W/ W v mutant mice and aged mice had the reduced amounts of JKTBP in the testes. Immunohistochemical staining indicated cell type-specific expression of JKTBP in tissues: neurons and spermatocytes displayed strong signal intensities. The signals were confined to the nucleus. The amount of 38 kDa JKTBP was estimated to be approximately 1.3×10 7 molecules per HL-60 cell. These results indicate that JKTBP is an abundant, highly conserved nuclear protein.