Rb correlates with tumorigenesis closely, and is an important tumor suppressor gene. Rb protein forms a complicated feedback regulation pathway combined with p16, CDK4/6, cyclinD1 to control cell cycle. Rb is a central step in G1/S checkpoint control to determine cell cycle progression. Rb protein acts as an interface of interaction between nuclear and extracellular signals and is controlled by many extracellular factors which control cell growth and differentiation.