An F3 site flanked neomycin resistance gene cassette and a loxP site were inserted into intron 3 and an FRT site flanked hygromycin resistance gene cassette and a second loxP site were inserted into intron 4. The neo and hygro cassettes were removed through subsequent flp-mediated recombination, leaving a conditional-ready allele with exon 4 floxed. (J:306088)