This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
The following is not one of the 4 key cell cycle regulators which are dysregulated in a variety of human cancers 
Select and explain. 
A. P16/INK4a, B. Cyclin D, C. P21, D. RB
C: P21
Answer: C: P21 
Explanation: .