Abstract: The present invention relates to nuclease protein fusions for enhancing genome editing by homology-directed transgene integration (HDI). The inventors found that the rate of HDI mediated by the CRISPR/Cas9 system may be substantially improved by providing the Cas9 nuclease in the form of a fusion protein with at least the N-terminal domain of the CtIP protein. CtIP proteins are involved in the early steps of homologous recombination. In addition, the inventors identified the subdomains of the N-terminal domain of the CtIP protein that are important for improving the HDI rate. Thus, the invention relates to fusion proteins comprising a Cas9 protein, a tetramerization domain of a CtIP protein and a dimerization domain of a CtIP protein. Particularly, the inventors have tested these fusion proteins HEK293 cells, RG37DR cells and Sprague-Dawley rats.