A loxP was inserted upstream of exon 20, and an FRT site, loxP site, neomycin cassette, FRT site, and loxP site were inserted downstream of exon 21. FLP-mediated recombination removed the neomycin cassette, leaving exons 20-21 floxed. Cre-mediated recombination deleted exons 20 and 21. (J:263824)