TN-L15 is a genetically encoded Ca[2+] indicator. It is a fusion protein composed of the ECFP and Citrine fluorescent proteins separated by separated by a calcium binding domain from the Gallus gallus TNNC2 gene (L15 variant which lacks the N-terminal 14 amino acid residues). Changes in fluorescence resonance energy transfer (FRET) between the two fluorophores can be used to detect changes in Ca[2+] levels (PMID:14742421). For a comparison of TN-L15 with other fluorescent Ca[2]+ indicators, see PMID:18447377.