A loxP site was inserted upstream of exon 3 and a neomycin resistance gene flanked by self-deletion anchor sites, a lox2272 site, a SA-2A-dTomato-polyA cassette in reverse orientation, a second loxP and lox2272 sites were inserted downstream of exon 3 in intron 3. The neomycin resistance gene was self-excised. Cre-mediated recombination inverted the dTomato cassette and exon 3 using the loxP sites, such that the dTomato cassette is now upstream of exon 3. (J:329964)