Cameleon(YC3.1) is a genetically encoded fluorescent Ca[2+] indicator. It is a fusion protein that consists of a cyan fluorescent protein (ECFP), calmodulin, the calmodulin-binding peptide M13 and a yellow fluorescent protein (EYFP-Q69K). A E104Q mutation is present within the calmodulin sequence. Binding of Ca[2+] to calmodulin causes it to bind M13, resulting in a conformational change that enhances the fluorescence resonance energy transfer (FRET) between the two fluorescent proteins (PMID:10051607).