Final answer:
p21 acts as a Cdk inhibitor or is controlled by p53. It binds to and inhibits the activity of the Cdk/cyclin complexes, preventing the progression of the cell cycle when there is damaged DNA.
Step-by-step explanation:
p21 acts as a Cdk inhibitor or is controlled by p53. p21 is a protein that enforces the halt in the cell cycle dictated by p53 by binding to and inhibiting the activity of the Cdk/cyclin complexes. This prevents the progression of the cell cycle, especially when there is damaged DNA.