Calcein AM readily passes through the cell membrane of viable cells because of its enhanced hydrophobicity as compared to calcein. The acetomethoxy (AM) derivate of calcein (calcein AM) is widely used for labeling live cells as it can be transported through the cellular membrane into live cells. The AM ester groups mask the part of the molecule that chelates calcium. Upon transporting into live cells cellular esterases cut off the AM groups, the molecule binds to calcium within cell (resulting in acquiring strong green fluorescence), and gets trapped inside. As dead cells lack esterases, only live cells are marked. This feature makes it very useful for testing of cell viability and for short-term marking of cells. Compared with other live cell-labeling reagents (such as BCECF-AM and carboxy-fluorescein diacetate), calcein-AM is the most suitable fluorescent probe for staining viable cells because of its low cytotoxicity. Calcein does not significantly affect cellular functions such as proliferation or chemotaxis of lymophocyte. In addition, viability assays using calcein are reliable and correlate well with the standard 51Cr-release assay.

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