A lox71 site, a neomycin resistance gene cassette and a lox2272 site were inserted into the 3' UTR in exon 4 immediately after the stop codon. Successfully targeted ES cells were transfected with a cre-expressing vector and a vector containing part of intron 74 of the human dystrophin (DMD) gene. This vector was constructed as follows: a lox66 site, genomic sequence from the last 11 bp of exon 74 up to the first 10 bp of exon 75, and a lox2272 site. The intronic sequence was shortened to 20 kb by deleting sequence from the central portion. Cre-mediated recombination recombined the genomic lox71 site with the vector lox66 site and the genomic and vector lox2272 sites with each other. This results in a knock-in allele with part of the human intron and parts of its flanking exons inserted into the 3' UTR intron of the host gene.