Marker gene

Selectable marker

Screenable marker

• Green fluorescent protein makes cells glow green under UV light. A specialized microscope is required to see individual cells. Yellow and red versions are also available, so scientists can look at multiple genes at once. It is commonly used to measure gene expression.
• GUS assay is an excellent method for detecting a single cell by staining it blue without using any complicated equipment. The drawback is that the cells are killed in the process. It is particularly common in plant science.
• Blue white screen is used in both bacteria and eukaryotic cells. The bacterial lacZ gene encodes a beta-galactosidase enzyme. When media containing certain galactosides, cells expressing the enzyme convert the X-gal to a blue product and can be seen with the naked eye. The strategy is therefore to integrate the DNA insert within the lacZ gene and to select the white colored colonies given they will have correctly integrated the insert. Blue colonies on the other hand will be able to convert the X-gal and give rise to the blue precipitate because the DNA insert either didn't integrate at all, or not at the correct location within the plasmid.