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 a cell cycle inhibitor 
Select and explain. 
A. P21, B. P27, C. P16/INK4a, D. Cyclin D-CDK4 complex
D: Cyclin D-CDK4 complex
Answer: D: Cyclin D-CDK4 complex 
Explanation: Checkpoints Cyclin CDK G1-S Cyclin D CDK 4/6 Cyclin E CDK 2 G2-M Cyclin A CDK 2/1 Cyclin B CDK 1 Family Checkpoints Proteins Functions CIP/KIP (CDKN1) G1-S G2-M p21 p27 p57 p21 induced by p53 p27 responds to TGF-ss INK4/ARF (CDKN2) G1-S p15 p16 p18 p19 p16/INK4a binds to cyclin D/CDK4 & promotes inhibitory effects of RB p14/ARF increased p53 levels by inhibiting MDM2