Twitch-2C is a genetically encoded ratiometric Ca[2+] indicator. It is a fusion protein composed of two fluorescent proteins (mTurquoise2 and a circularly permuted form of Citrine) separated by a minimal calcium binding domain derived from the Opsanus tau troponin C gene (this domain has been mutated to contain the K14F, N15D, D17N, N51D, D53N and M65V amino acid substitutions and is linked to the fluorophores at its N-terminal and C-terminal ends by a VADA and a PIYP linker respectively). Changes in fluorescence resonance energy transfer (FRET) between the two fluorophores can be used to detect changes in Ca[2+] levels (PMID:24390440). For a comparison of Twitch-2C with other fluorescent Ca[2]+ indicators, see PMID:24390440.