An F3 recombinase recognition sequence followed by an FRT flanked neo cassette was inserted in intron 2 and a floxed puro cassette followed by a second F3 sequence was inserted in intron 3. At the same time, a mutation was inserted in exon 3 which causes a change in codon 63 resulting in a serine to alanine substitution (S63A). Subsequent cre and flp expression excised the two selection cassettes and left exon 3 flanked by F3 sites. (J:191723)