Disclosed is a method for introducing a mutation into a gene, which comprises the following steps: a homologous recombination step of carrying out the homologous recombination between a target gene into which the mutation is to be introduced and a target recombinant vector, thereby substituting an exon in the target gene into which the mutation is to be introduced by a target DNA sequence in the target recombinant vector and a mutation introduction step of carrying out the specific recombination between the target DNA sequence in the resulting target recombinant gene and a mutation introduction cassette of a mutation introduction vector carrying a mutated DNA sequence containing a mutant exon by the intervening action of Cre recombinase to substitute the target DNA sequence by the mutated DNA in the mutation introduction cassette, thereby producing a mutation-introduced gene into which the mutant DNA sequence has been introduced. The method enables the production of a knock-in non-human mammalian animal, such as a knock-in mouse, which carries the mutation-introduced gene.
