This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
During growth factor-induced cellular regeneration, which transition during cell cycle is controlled by the phosphorylation of the retinoblastoma protein: 
Select and explain. 
A. G0 to Gl, B. Gl to S, C. S to G2, D. G2 to M
B: Gl to S
Answer: B: Gl to S 
Explanation: Rb proteins control any extremely impoant check point, the Gi to S transition during the cell cycle. The other checkpoints are regulated by a distinct set of proteins.