TN-XXL is a genetically encoded ratiometric Ca[2+] indicator. It is a fusion protein composed of two fluorescent proteins (ECFP and a circularly permuted form of Citrine) separated by calcium-binding domains from the Gallus gallus TNNC2 gene (in TN-XXL the N-terminal calcium-binding lobe has been deleted and replaced by an additional copy of the C-terminal lobe and each copy of the C-terminal lobe contains three amino acid replacements; N108D, D110N and I130T). Changes in fluorescence resonance energy transfer (FRET) between the two fluorophores can be used to detect changes in Ca[2+] levels (PMID:18447377). For detailed kinetic and spectral properties and a comparison of TN-XXL with other fluorescent Ca[2]+ indicators, see PMID:18447377.