The protein Calmodulin is a ubiquitous intracellular calcium sensor that responds to changes in calcium concentration. Each Calmodulin molecule has four binding sites for Ca2+. The molecule undergoes a conformational change after calcium- binding. This conformational change alters the affinity of Calmodulin towards its various target proteins, whose functions are regulated upon Calmodulin binding.

By protein engineering a chimeric fusion protein between CFP, Calmodulin, the M13 peptide (Calmodulin binding domain of myosin light chain kinase M13) and YFP can be created. Such a fusion protein is called Cameleon after the original calcium ion probe developed in 1997 by Dr. Atsushi Miyawaki. Without binding calcium the molecule is stretched (no FRET between CFP and YFP). The conformational change of Calmodulin after binding calcium causes an interaction with the M13 peptide. The overall molecular motion brings CFP and YFP into close proximity and permits the emission of a FRET signal.