Fig. 1. The Cdk1^Cdk2KI knockin construct. (A) Schematic showing the strategy used to generate the Cdk1^Cdk2KI knockin construct (for details see Materials and methods). Exons (red arrowheads) are numbered. (B,C) Mouse embryonic stem (ES) cell clones positive for the Cdk1^Cdk2KI locus were screened for lacZ expression (β-galactosidase staining, blue) (B) and by PCR (C). Exon1^Cdk2-Cdk2 indicates endogenous Cdk2, whereas Exon1^Cdk1-Cdk2 indicates that exon 1 was from Cdk1 and the remaining coding region from Cdk2. Three independent embryonic stem cell clones (1, 2, 3) are shown that had the Cdk1^Cdk2KI locus correctly targeted. (D) Western blots and histone H1 kinase assays indicating the expression level of knockin Cdk2-HA (top panel) and Cdk2 (bottom panel), and the kinase activity of knockin HA-tagged Cdk2 (second panel from top) and of endogenous Cdk2 (third panel from top) in Cdk2^-/- (lane 1), Cdk2^+/+ (lane 2) and Cdk2^+/+ Cdk1^+/Cdk2KI (lane 3) MEFs.