New observations by Hatini et al. on the 'winged helix' transcription factor BF-2 will make us change our views about kidney development. This gene is only expressed in stromal cells associated with the kidney medulla and cortex, but the BF-2 knockout has unexpected abnormalities. Although the stromal cells appear normal, the kidney is small, the ducts have limited branching and, instead of the many normal nephrogenic aggregates, there are relatively few large mesenchymal aggregates that fail to differentiate. The stromal cells thus seem to produce factors regulating other aspects of kidney development, while the abnormalities highlight a series of unsolved problems in kidney morphogenesis.