Figure 1.

Maintenance of the G₁/S DNA damage checkpoint in the presence and absence of Cdk2. In response to DNA damage, activation of p53-p21 pathway is not altered in the absence of Cdk2, the primary target of p21 at the G₁/S checkpoint. In the presence of Cdk2, the induced p21 inhibits Cdk2/cyclin E complexes in response to DNA damage. In the absence of Cdk2, Cdk1/cyclin E complexes are responsible for promotion of the G₁/S transition and as a result become the target for p21 inhibition in response to DNA damage. Nevertheless, Cdk1 is not fully capable to rescue the functions of Cdk2 in DNA damage repair.