Clomeleon is a ratiometric reporter of changes in intracellular chloride concentration. It consists of a CFP derivative fused via a flexible peptide linker to the Topaz yellow fluorescent protein (which can bind to Cl[-]). Changes in Cl[-] levels can be detected as a change in the efficiency of fluorescence resonance energy transfer (FRET) between the CFP derivative (FRET donor) and Topaz (FRET acceptor). The peptide linker includes a Tag:CS(TEVp) cleavage sequence (PMID:11055428).