A neomycin resistance gene flanked by loxP sites was inserted into intron 3 while a third loxP site was inserted into intron 1, generating Cdk2tm2Sgo. The present allele resulted from Cre recombinase-mediated excision of exons 2 and 3 (and the selection cassette), leading to a predicted reading frame shift. Gene modification was confirmed by RT-PCR and sequencing of the resulting PCR product.