By homologous recombination in embryonic stem cells (ES), two loxP sites and a neo cassette were introduced flanking the exon 2, one at 212 bp upstream of exon 2 and another at 53 bp downstream of the exon 2 after the Neomycin resistance cassette, which was flanked by two FRT sites. The neo cassette was removed by FLIP-recombination. (J:298868)