Microbiologists traditionally determine which organisms they are dealing with using the small subunit ribosomal RNA or in short SSU rRNA gene. This marker gene allows them to identify almost any living creature, be it a bacterium or an animal, and thus assign it to its place in the tree of life. Once the position in the tree of life is known, specific DNA probes can be designed to make the organisms visible in an approach called FISH (fluorescence in situ hybridization). FISH has many applications, for example to sort cells, or to microscopically record their morphology or spatial position. This approach—which leads from DNA to gene to tree and probe to image—is called the 'full-cycle rRNA approach.'