A strain of E. coli is genetically engineered in which the lacZ and lacI genes are removed and replaced with a gene encoding a fluorescent protein. A copy of the operon in which the protein product fluoresces green is inserted into the chromosome, and a copy in which the protein product fluoresces red is inserted into a plasmid. Both copies carry a wild-type lacO gene, and the cells of the strain have a single copy of the lacI repressor gene. If both fluorescent proteins are expressed, the cells fluoresce yellow (because a combination of red and green fluorescence appears as yellow), and if neither of the fluorescent proteins is expressed, the cells show no fluorescence. The cells would fluoresce in the presence of an inducer and in the absence of an inducer.