Cell growth occurs through an ordered sequence of events : The cell cycle, which consists of four distinct phases, G1, S, G2 and M phase. In February 2008 Atsushi Miyawaki from the RIKEN Basic Science Institute reported that his group has developed a method, Fucci that will allow cell cycle researchers to visualize cell cycle progression. Fucci (fluorescent, ubiquitination-based cell cycle indicator) modified cells are yellow at the start of replication, switch to green during S phase and to red during G1.

To demonstrate the utility of Fucci the researchers created a Fucci mouse. The colored images shown on the right demonstrates the equilibrium between cell differentiation and cell proliferation that occurs during the development of the mouse head. The red and green signals appear to be well balanced at the embryonic stage, but the overall ratio of green-to-red signal decreases as the mice grow.