We have previously reported the cloning, sequencing, and partial characterization of Kid-i, a zinc finger-encoding cDNA from the rat kidney. A limited number of structural motifs have been described for eukaryotic transcription factors. A common motif is the zinc finger structure, which is highly conserved and found in many different species (1). Though the crystal structure of a cocrystal between the three zinc fingers of zif-268 (Egr-i) and their binding site has been elucidated (2), little is known about the interaction between zinc fingers and DNA and how these protein-DNA interactions are involved in the regulation of transcription.Approximately one-third of all zinc finger proteins contain an evolutionarily conserved region of about 75 amino acids at their NH2 terminus-the Krippel-associated box (KRAB), a sequence motif of hitherto unknown function. Similar to regions found in a subset of homeodomain proteins (the paired box and POU domain), the KRAB domain is rich in charged amino acids (3). Because of the potential a-helical structure of KRAB domains, it has been proposed that this domain mediates protein-protein interactions and functions as a transcriptional regulatory domain (3), but this hypothesis has not been previously tested.