The engineering of protein-DNA interactions in different protein scaffolds may provide "toolkits" to modify the genome. Homing endonucleases are powerful tools for genome manipulation through homologous recombination, as these enzymes possess a very low frequency of DNA cleavage in eukaryotic genomes due to their high specificity. Therefore, the combination of a precise "cutter" with the presence of a natural or modified homologous DNA donor provides a potentially useful means to modify the genome. However, the basis of protein-DNA recognition must be understood to generate tailored enzymes that target the DNA at sites of interest. The engineering of homing endonucleases and alternative scaffolds, such as zinc fingers or transcription activator-like effector domains, has demonstrated the potential of these approaches to create new specific instruments to target genes for inactivation or repair. Customized homing endonucleases targeting selected human genes can excise or correct regions of genes implicated in monogenic diseases, thereby representing important tools for intervention in eukaryotic genomes.