A construct containing an upstream FRT site, coding sequence for the Cdk2 protein that excludes the first 12 amino acids and includes a carboxy-terminus HA tag, an IRES-beta-galactosidase gene, another FRT side and a loxP-flanked neomycin gene was used to target exon 3 of the gene locus. The insertion site maintains all the exon 3 splicing sequences of the endogenous locus. This targeted mutation resulted in a transcript that fused the 5'UTR encoded by exon 1 and the ATG start codon plus 11 amino acids encoded by exon 2 to the coding sequence of the inserted gene. The first 12 amino acids of the endogenous gene and the knock-in are very similar (they differ by four amino acids- N3D, F4Y, Q5I and V7I), suggesting that this region would not affect the Cdk2 knock-in properties. Lysates of mouse embryonic fibroblasts isolated from heterozygote mice indicated that the knock-in Cdk2 was expressed and displayed kinase activity similar to that of endogenous Cdk2. In addition, extensive Beta-galactosidase staining was observed in the spermatocytes of heterozygous mice.