A single targeting event was used to insert a floxed Bsd resistance cassette between exon 2 and 3 of Il13 and an frt flanked neo cassette with a 3' loxP site downstream of exon 4 of Il4 . Flp and cre mediated recombination removed the selection cassettes leaving the region between exon 3 of Il13 and exon 4 of Il4 floxed.