The present invention relates to a method of producing a non-human, mammalian oocyte carrying a modified target sequence in its genome, the method comprising the steps of introducing into a non-human, mammalian oocyte: (a) a clustered, regularly interspaced, short palindromic repeats (CRISPR)-associated protein 9 (Cas9 protein) or a nucleic acid molecule encoding said Cas9 protein; and (b-i) a target sequence specific CRISPR RNA (crRNA) and a trans-activating crRNA (tracr RNA) or a nucleic acid molecule encoding said RNAs; or (b-ii) a chimaeric RNA sequence comprising a target sequence specific crRNA and tracrRNA or a nucleic acid molecule encoding said RNA; wherein the Cas9 protein introduced in (a) and the RNA sequence(s) introduced in (b-i) or (b-ii) form a protein/RNA complex that specifically binds to the target sequence and introduces a single or double strand break within the target sequence. The present invention further relates to the method of the invention, wherein the target sequence is modified by homologous recombination with a donor nucleic acid sequence further comprising the step: (c) introducing a nucleic acid molecule into the cell, wherein the nucleic acid molecule comprises the donor nucleic acid sequence and regions homologous to the target sequence. The present invention also relates to a method of producing a non-human mammal carrying a modified target sequence in its genome.