Final answer:
Cdk inhibitors, such as p21, are molecules that prevent the full activation of Cdks in the cell cycle and block its progression until specific events are completed.
Step-by-step explanation:
Cdk inhibitors are molecules that prevent the full activation of Cdks in the cell cycle. They play a crucial role in blocking the progression of the cell cycle until specific events are completed. One example of a Cdk inhibitor is p21, also known as cyclin-dependent kinase inhibitor 1.