A loxP site and an FRT site flanked neomycin resistance gene cassette were inserted into intron 1. An F3 site flanked puromycin resistance gene cassette and a second loxP site were inserted into intron 2. The neo and puro cassettes were removed through subsequent flp-mediated recombination, leaving a conditional-ready allele with a floxed exon 2. (J:240754)