is an rna binding domain this domain recognizes and interacts with the protein binding site in the rna discussed above in a specific binding interaction under physiological conditions a preferred rna binding domain is derived from the bacteriophage ms2 coat protein capsid which binds with high specificity to a unique site on ms2 rna see fiers et al supra valegard et al supra fouts et al supra and sengupta et al supra other proteins containing rna binding domains are discussed above in some embodiments the fusion protein includes a third domain that causes intracellular localization of the fusion protein when the fusion protein is not bound to its rna target examples of intracellular localization domains include nuclear localization signal nls domains nuclear export signal nes domains and nucleolar targetting domains various nls sequences are known in the art any of which can be used in the invention a preferred nls is derived from sv40 a well known simian virus a useful nes can be derived from human immunodeficiency virus hiv rev sequences when the fusion protein contains an nes domain fluorescence is detected inside the nucleus where the protein is bound to its target rna in cases where the rna is undergoing biosynthesis at the site of its gene transcription site the fluorescence is most intense at the transcription site thus the site of active transcription of a specific gene in a living cell is visualized a fusion.