The FISH technique is dependent upon hybridizing a probe with a fluorescent tag, complementary in sequence, to a short section of DNA on a target gene. The tag and probe are applied to a sample of interest under conditions that allow for the probe to attach itself to the complementary sequence in the specimen if it is present. After the specimen has been treated, excess fluorophore is washed away and the sample can be visualized under a fluorescent microscope. By quantifying the amount of fluorescence with the scope it can be determined if the type of cell the probe was designed for is present, and if so, how much of it is present in a sample. Transcripts (mRNA) in microbes can also be targeted to detect if a specific gene is being expressed under the given conditions.