Calcein

Calcein (Fluorexon) is a fluorescent fluid phase marker used to track and visualize cellular processes such as synaptic vesicle fusion. Calcein is also the fluorophore for live cells in the commonly used Live/Dead viability assay. (Calcein itself cannot penetrate the cell membrane and cannot be directly applied in experiments assessing live cells. In practice, Calcein AM is employed, which can enter cells and be hydrolyzed by intracellular enzymes, releasing the fluorescent Calcein compound, thereby achieving the labeling of live cells.)