Cameleon(D3mC3+16) is a ratiometric fluorescence resonance energy transfer (FRET)-based Ca[2+] indicator. It consists of two fluorophores, mCer3 and a circularly permuted form of Venus, joined by a Ca[2+] sensitive module consisting of mutant forms of calmodulin and a calmodulin binding peptide (these two components are separated by a flexible linker of 16 Gly residues). For detailed kinetic and spectral properties, see PMID:31203189.