Zinc-finger nucleases (ZFNs) are promising new tools for enhancing the efficiency of gene targeting in many organisms. Because of the flexibility of zinc finger DNA recognition, ZFNs can be designed to bind many different genomic sequences. The double-strand breaks they create are repaired by cellular processes that generate new mutations at the cleavage site. In addition, the breaks can be repaired by homologous recombination with an exogenous donor DNA, allowing the experimenter to introduce designed sequence alterations. We describe the construction of ZFNs for novel targets and their application to targeted mutagenesis and targeted gene replacement in Drosophila melanogaster and Caenorhabditis elegans .