Final answer:
The protein p21 is an example of a Cdk inhibitor protein that is regulated by the tumor suppressor gene p53 and plays a critical role in inhibiting the cell cycle progression.
Step-by-step explanation:
An example of a Cdk inhibitor protein is p21. Cdk inhibitors are proteins that can inhibit the activity of cyclin-dependent kinases (Cdks) and are regulated by tumor suppressor genes such as p53.
P21, which is controlled by p53, is one such inhibitor, and when p53 is activated in response to DNA damage or other cellular stress signals, it can lead to the expression of p21. The role of p21 is to bind and inhibit the activity of Cdk/cyclin complexes, preventing them from phosphorylating target proteins that are necessary for the progression of the cell cycle.