What is calcein AM?

Calcein is a fluorescent dye with excitation and emission wavelengths of 495/515 nm, respectively, which cannot penetrate the plasma membrane of viable cell due to the negatively charged nature. Calcein AM (acetoxymethyl), on the other hand, is the non-fluorescent acetomethoxy derivate of calcein, whose carboxylic groups are protected by the AM esters, making the dye neutral and permeable to the membrane live cells. Once inside the cell, active esterases will cleave AM groups, and the dye molecule is trapped inside the cell and gives out strong green fluorescence. Calein AM is widely used in cell viability assays and for short-term labeling of cells.