Final answer:
p21, a Cdk inhibitor protein controlled by p53, specifically inhibits G1/S-Cdks to halt the cell cycle at the G1 checkpoint in response to DNA damage.
Step-by-step explanation:
The Cdk inhibitor proteins produced by p21 stop the progression of the cell cycle at the G1 checkpoint by binding to and inhibiting the activity of Cdk/cyclin complexes. The specific type of Cdks that p21 inhibits are G1/S-Cdks, which are involved in the transition from G1 phase to S phase of the cell cycle. p21 is also controlled by p53, which is a tumor suppressor protein that triggers the production of p21 in response to DNA damage. Consequently, p21 acts to enforce the halt in the cell cycle dictated by p53 by preventing the activation of the Cdk/cyclin complexes required for the cell to progress into the S phase.