A loxP site was inserted upstream of exon 1 and a floxed neomycin cassette was placed downstream of exon 1. Cre recombinase was transiently expressed in correctly targeted ES cells to remove the neomycin selection cassette, leaving exon 1 flanked by loxP sites. Genomic PCR and Southern blotting confirmed removal of the neo cassette and the flox-state of exon 1. (J:95215, J:144110)