Congenital nephrotic syndrome of the Finnish type (CNF or NPHS1) is an autosomal recessive kidney disorder resulting in severe proteinurea and renal dysfunction. Although the disease occurs predominantly in the Finnish population, many cases in other populations have also been reported. The disease gene (NPHS1) encodes nephrin, a podocyte transmembrane protein that is an essential component of the podocyte slit diaphragm, the renal ultrafilter. Since the discovery of the gene, many mutations have been reported in the NPHS1 gene in patients with diverse ethnic background. A surprisingly large number of these mutations are missense mutations resulting in single amino acid substitutions. In order to study the pathomechanism of these missense mutations, we have investigated the fate of 21 such mutations hitherto identified in NPHS1 patients. Immunostaining of stable transfected cells expressing the nephrin mutants demonstrated that most of the mutants showed only endoplasmic reticulum (ER) staining and no detectable cell surface localization. Immunoelectron microscopy of cells expressing the wild-type and a mutant nephrin further confirmed that the mutant nephrin could be abundantly found in the ER but not on the plasma membrane. Subcellular fractionation of wild-type and a mutant cell line clearly showed an altered subcellular distribution and molecular mobility of the mutant nephrin. In summary, our data indicate that a defective intracellular nephrin transport, most likely due to misfolding, is the most common consequence of missense mutations in NPHS1.