Calcein

Calcein, also known as fluorexon, fluorescein complex, is a fluorescent dye with an excitation and emission wavelengths of 495/515nm, respectively. Calcein also self-quenches at concentrations above 100mM. It is used as a complexometric indicator for titration of calcium ions with EDTA, and for fluorometric determination of calcium. It has the appearance of orange crystals.

The acetomethoxy derivate of calcein (calcein AM) is used in biology as it can be transported through the cellular membrane into live cells, which makes it useful for testing of cell viability and for short-term marking of cells. The acetomethoxy group obscures the part of the molecule that chelates calcium. After transport into the cell the enzymes cut off the group, the molecule binds to calcium within cell (resulting in strong green fluorescence), and gets trapped inside. As dead cells lack this enzyme, only live cells are marked. 

Calcein is also used for marking freshly hatched fish and for labeling of bones in live animals.