Carboxyfluorescein succinimidyl ester (CFSE) is used for tracking cells in vivo and also tracing their mitotic activity. This fluorescent dye stains intracellular proteins and generates a fluorescent signal that progressively halves with each mitosis. Reduction in fluorescence intensity can be quantified by flow cytometry (FCM) and an algorithm used to evaluate the extent of blastogenesis. Furthermore, cultured cells can be stained for expression of other cell surface markers to define lineage or activation state. This approach has the advantage of allowing specific gating on particular T cell subsets and simultaneous assessment of activation markers[1]