An frt site was inserted 230 bp upstream of exon 2 and a frt-flanked hygromycin resistance cassette 115 bp downstream of exon 4 in ES cells already carrying the Cdk2tm2.1Sgo conditional mutation of Cdk2 and integrated into the same chromosome. The selection cassette was excised by Flp-mediated recombination, leaving exons 2-4 flanked by frt sites.