5- or 6-(N-Succinimidyloxycarbonyl)-fluorescein 3',6' diacetate
CFSE is cell-membrane permeable and readily accumulates inside viable cells where it covalently attaches to intracellular proteins (Fig. 1). Hydrolyzed CFSE emits fluorescence and covalently attached fluorescein molecules do not leak from cells. CFSE-labeled cells can be monitored over several weeks in vivo. Therefore, CFSE is utilized for detection of viable cell as well as for the long-term observation of cell activities by fluorescent microscopy.