In accordance with the present invention, there are provided cells for expressing recombinase Cre in the presence of recombinase FLP in a FLP-dependent manner, methods of expressing recombinase Cre by introducing recombinase FLP into the above cells, methods of producing recombinant viral vectors using cells that express recombinase Cre in the presence of recombinase FLP in a FLP-dependent manner, and methods of producing recombinant adenovirus vectors using the above method of producing Cre and the above method of producing recombinant viral vectors.