Calcium is a universal second messenger regulating essential cellular signaling events in a broad range of tissues and organisms. GCaMP3 is a fusion calcium indicator of green fluorescent protein (GFP), calmodulin and the M13 domain of a myosin light chain kinase designed to show the calcium ion status in vivo. When calcium is present, fusion calcium indicator undergoes a conformational change and GFP fluorescence is activated from quenched status. GCaMP3 has high signal-to-noise ratio, faster kinetics and greater response and has been applied as ideal image technique to monitor neuron action potentials, which would otherwise require large numbers of electrodes or potentiometric imaging.