Y-GECO2.1s is a genetically encoded fluorescent Ca[2+] indicator with an inverted response to Ca[2+] (fluorescence decreases upon binding Ca[2+]). It is a fusion protein composed of the M13 calmodulin-binding peptide, a circularly permuted yellow fluorescent protein and calmodulin. Y-GECO2.1s was derived by mutagenesis of Y-GECO1m, containing in addition the mutations Q276D and M300I. For detailed kinetic and spectral properties, see FBrf0239772.