A loxP site was placed 1.9 kb upstream of the first exon. A second loxP site was placed in the first intron. A puromycin resistance (PuroR) gene flanked by F3 sites was inserted into intron 1 upstream of the second loxP site for positive selection. The puromycin resistance (PuroR) gene was removed via flp1-mediated recombination and exon 1 was removed via cre-mediated recombination in the germline.