The present invention provides a novel modified protein which is to be used, as a novel detection tool relating to gene expression, for detecting a chromatin open structure more easily at a higher sensitivity than by the conventional technique. The present invention relates to: a nucleic acid binding fluorescent protein, said protein containing a DNA binding domain in which 3 or more TAL-repeats are repeatedly connected, characterized by binding independently from base sequences; and a method for fluorescent labeling of an open chromatin in a vital cell, said method comprising a step for transferring a gene encoding a nucleic acid binding protein into the vital cell, characterized in that the nucleic acid binding protein is a protein comprising a DNA binding domain, in which 3 or more TAL-repeats are repeatedly connected, and a fluorescent protein directly or indirectly bound thereto and the DNA binding domain binds to a nucleic acid independently from base sequences.