Cameleon(D1) is a ratiometric fluorescence resonance energy transfer (FRET)-based Ca[2+] indicator. It consists of two fluorophores (a truncated version of ECFP and Citrine) joined by a Ca[2+] sensitive linker consisting of mutant forms of calmodulin and a calmodulin binding peptide derived from skeletal muscle myosin light chain kinase. High Ca[2+] levels induce a conformational change which increases FRET between the fluorophores and strengthens the Citrine signal (PMID:15585581).