Individual zinc finger (ZF) domains that recognize DNA triplets with high specificity and affinity can be used to create designer transcription factors and nucleases that are specific for nearly any site in the genome. These domains can be treated as modular units and assembled to create a polydactyl protein that recognizes extended DNA sequences. Deter-mination of valid target sites and the subsequent design of ZF proteins (ZFPs) is error-prone and not trivial, however. As a result, the use of ZFPs have been restricted primarily to those labs with the appropriate expertise. To address these limitations, we have created a user-friendly utility called Zinc Finger Tools (ZF Tools) that can be accessed at the URL http://www.zincfingertools.org. User-supplied DNA sequences can be searched for target sites appropriate for either gene regulation or nuclease targeting. Using a database of experimentally characterized zinc finger domains, the amino acid sequence for a ZFP expected to bind to any chosen target site can be generated. A reverse engineering utility is provided to predict the binding site for a ZFP of known sequence.