Calcein AM is a cell-permeable dye that, upon entering live cells, is cleaved by intracellular esterases, leaving membrane-impermeant calcein, a fluorescent indicator with absorption and emission maxima of 494 and 517 nm, respectively. It can be used as an indicator of cell viability, cell-cell communication, cytotoxicity, or changes in intracellular calcium, fluoride, iron, or mercury. As calcein is a substrate for multidrug resistance-associated protein 1 (MRP1) and multidrug resistance protein 3 (MDR3, P-glycoprotein 3), calcein AM is used to study these proteins and their regulation.