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 transition from G2 to M phase is controlled by 
Select and explain. 
A. Rb gene product, B. P53 protein, C. Cyclin E, D. Cyclin B
D: Cyclin B
Answer: D: Cyclin B 
Explanation: The cell cycle is regulated by activators and inhibitors. Cell cycle progression is driven by proteins called cyclins. cyclin D-CDK4, cyclin D-CDK6, and cyclin E-CDK2 regulate the G1-to-S transition by phosphorylating the Rb protein (pRb). Cyclin A-CDK2 and cyclin A-CDK1 are active in the S phase. Cyclin B-CDK1 is essential for the G2-to-M transition.Reference: Robbins and Cotran Pathologic Basis of Disease; 9th edition; Chapter 1; The Cell as a Unit of Health and Disease