The succinimidyl ester of carboxyfluorsecein diacetate [5(6)]- CFSE is the best reagent currently available for the analysis of cellular proliferation. CFSE spontaneously and irreversibly couples to both intracellular and cell surface proteins by reaction with lysine side chains and other available amine groups. When cells divide, CFSE labeling is distributed equally between the daughter cells, which are therefore half as fluorescent as the parents. As a result, halving of cellular fluorescence intensity marks each successive generation in a population of proliferating cells and is readily followed by flow cytometry. The number of divisions, which can be followed, is limited only by the auto fluorescence level of unlabeled cells, and the uniformity in size of the labeled cell population (Hodgkin et al., 1996).