Filtration of plasma water and solutes is extracellular and occurs through the endothelial fenestrae and filtration slits. The importance of the podocytes and the filtration slits is shown by genetic diseases. Thus, in congenital nephrotic syndrome of the Finnish type, the gene for nephrin, a protein of the filtration slit, is mutated, leading to nephrotic syndrome in infancy . Similarly, podocin, a protein of the podocytes, may be abnormal in a number of children with steroid-resistant focal glomerulosclerosis.