The hb canonical promoter from -300bp to +445bp relative to the transcription start site, which includes the hb intron, is fused upstream of the iRFP670 coding sequence. 24 copies of the Tag:MS2 stem loop, in which all 36 putative zld protein-binding sites have been mutated (these potential binding sites were all located in linkers between the Tag:MS2 loops), have been inserted downstream of the coding sequence. In addition, the unique putative zld protein-binding site in the iRFP670 sequence has been mutated.