Two loxP sequences flanking conserved non-coding sequence 3 (CNS3 (Rr131)), within intron 3 downstream of the 1st coding exon, as well as an FRT site flanked neomycin resistance gene cassette were inserted by homologous recombination. In addition, an EGFP cassette was inserted in-frame into the first coding exon. The selection cassette and the CNS were removed by flp- and cre-mediated recombination.