The concentration of calcium in living cells can be measured using differently coloured green fluorescent protein (GFP) mutants. Two groups have independently linked blue-shifted GFP mutants to longer-wavelength GFP mutants through connecting sequences to which Ca2+-calmodulin or Ca2+ alone can bind. Fluorescence energy resonance transfer occurs between the two mutants, but it is very sensitive to the distance and orientation of the fluorophores, and so can detect Ca2+ binding.
