TN-XL 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 a calcium binding domain from the Gallus gallus TNNC2 gene (XL variant which contains the mutations N109D, D111N, N145D and D147N relative to the L15 variant). Changes in fluorescence resonance energy transfer (FRET) between the two fluorophores can be used to detect changes in Ca[2+] levels (FBrf0189879). For a comparison of TN-XL with other fluorescent Ca[2]+ indicators, see PMID:18447377.