Genetic approaches have clarified the molecular basis of many different stages of kidney development with considerable clarity. By introducing targeted mutations in mice, a number o transcription growth factors have been shown to be required for early kidney tubulogenesis. Most recently, the transcription factor Pax-2, and bone morphogenetic protein-7, a growth factor, were added to the list of factors required for the early stages of kidney tubulogenesis. Compared with the defects seen in mice lacking Pax-2 or bone morphogenic protein-7, the formation of cysts in tubules is morphologically a very mild defect. Cysts are seen in many transgenic mice with overexpression of a gene in the kidney, and in some 'knockout' mice lacking a gene. Some of these genes might be involved in human cystic diseases. However, it was recently shown that the gene affected in 85% of patients with autosomal dominant polycystic kidney disease encodes a novel protein, called polycystin. This protein is very large and has a sequence suggesting multiple transmembrane domains. It extracellular domains suggest that polycystin is involved in cell-cell and cell-matrix interactions.