Case12 is a Ca[2+] sensor composed of a circularly permuted fluorescent protein that is tagged at the N-terminal end with an M13 peptide and at the C-terminal end with a CaM Ca[2+] binding sequence. Ca[2+]-binding results in a change in conformation of the chromophore, which can be detected as a change in the level of fluorescence (a 12-fold increase in fluorescence is seen in response to Ca[2+]). The protein shows fast maturation and high brightness (PMID:17603870).