The cell cycle is a complex mechanism that governs cell growth and proliferation. Cell proliferation contributes to the continuity of life by producing cells, replenishing cells that have undergone cellular differentiation until acquiring specialized phenotypes (function and morphology) to carry out the living mechanism and towards final cell death . Cell proliferation is determined both by extracellular signals such as cytokines and mitogens, and by intrinsic cellular factors. Interactions of extracellular signals with intrinsic cellular factors trigger the biochemical events of cell proliferation. In the case of acquired immunity, proliferation is the important state after lymphocytes encounter until antigen presentation, and then leads to their effector functions. Cell cycle regulators control proper entry and progression throughout the cell cycle event. Thus, any deregulation of the cell cycle will potentially lead to tumorigenesis. (Malumbres and Carnero 2003) Upon exposure to extracellular signals and activation of intrinsic cellular factors, cells undergo an ordered series of o...