Cameleon(D4cpv) is a low-affinity, ratiometric fluorescence resonance energy transfer (FRET)-based Ca[2+] indicator. It consists of two fluorophores, ECFP (truncated by 11 residues) and a circularly permuted form of Venus, joined by a Ca[2+]-sensitive element composed of mutant forms of calmodulin and a calmodulin-binding peptide. The mutations change the Ca[2+] affinity and prevent perturbation by wild-type calmodulin (PMID:16720273).