As a calcium indicator protein having an excellent fluorescent characteristic and calcium reactivity, there is provided DNA in which one of a nucleotide sequence derivative of a calmodulin-binding sequence (ckkap sequence) of calcium/calmodulin-dependent protein kinase kinase and a nucleotide sequence encoding a calcium-binding sequence (CaM sequence) of calmodulin is linked to a 5' end of a nucleotide sequence encoding a fluorescent protein, and the other nucleotide sequence is linked to a 3' end of the nucleotide sequence encoding the fluorescent protein. The calcium indicator protein encoded by this DNA, which based on the derivative of the ckkap sequence as a binding domain for the calcium-bound CaM sequence, exhibits a fluorescent characteristic and calcium reactivity superior to those of conventional calcium indicator proteins.
