Nephrin, a gene product of the congenital nephrotic syndrome of the Finnish type (NPHS1), is a 1242-residue putative transmembrane protein of the immunoglobulin family of cell adhesion molecules. The expression of this gene is localized in rat and human glomerular epithelial cells. Here we report the expression of nephrin in various tissues other than the kidneys in mice. The expression of nephrin mRNA in various tissues of mice, including the kidneys, testes, spleen, thymus and brain, were first investigated by the RT-PCR method, and it was shown that a high level of nephrin mRNA could be detected in the testes of mice 1-6 weeks old. In situ hybridization revealed the expression of the nephrin gene in the Sertoli cells. Additionally, immunofluorescent staining studies indicated that nephrin was colocalized with anchoring protein ZO-1 in the mouse testis. From these results, it is inferred that nephrin is an important component of the barrier system in testes.