Factors regulating the temporal and spatial expression of kidney developmental programs are beginning to be identified. Evidence accumulated in the past few years indicates that the Hox and Pax genes, in particular, are important for early kidney development. A number of soluble growth factors were also shown to be essential for nephrogenesis to proceed in vitro. Errors in kidney development include Wilms tumor, a common childhood malignancy, which in many cases seems to be caused by faulty transcriptional regulation.