Fura-2-acetoxymethyl ester, often abbreviated Fura-2AM, is a membrane-permeable derivative of the ratiometric calcium indicator Fura-2 used in biochemistry to measure cellular calcium concentrations by fluorescence. When added to cells, Fura-2AM crosses cell membranes and once inside the cell, the acetoxymethyl groups are removed by cellular esterases. Removal of the acetoxymethyl esters regenerates "Fura-2", the pentacarboxylate calcium indicator. Measurement of Ca2+-induced fluorescence at both 340 nm and 380 nm allows for calculation of calcium concentrations based 340/380 ratios. The use of the ratio automatically cancels out certain variables such as local differences in fura-2 concentration or cell thickness that would otherwise lead to artifacts when attempting to image calcium conc