Method for producing transformant

A method for producing a transformant comprising steps of:

• i) introducing into at least one host cell a first group of three or more nucleic acid fragments having homologous recombination sequences each located at their 5′ and 3′ ends,
• wherein the first group of nucleic acid fragments comprises:
• a first nucleic acid fragment comprising a target gene, wherein the target gene is a selection marker gene;
• a second nucleic acid fragment in which the homologous recombination sequence at 5′ end is either one of a pair of homologous recombination sequences corresponding to a particular region of genome DNA;
• and
• a third nucleic acid fragment in which the homologous recombination sequence at 3′ end is the other of the pair of homologous recombination sequences;
• wherein the nucleic acid fragments constituting the first group of nucleic acid fragments can be linked to each other via the homologous recombination sequence at the end of each nucleic acid fragment, and
• wherein each of a pair of recognition sequences recognized by a site-specific recombinase is comprised in nucleic acid fragments among the first group of nucleic acid fragments, but none of the pair of recognition sequences is comprised in the first nucleic acid fragment comprising the target gene;
• ii) selecting a host cell in which the target gene flanked by the pair of recognition sequences recognized by the site-specific recombinase had been cleaved from genome DNA.