Marker Systems

Marker systems are tools for studying the transfer of genes into an experimental organism. In gene transfer studies, a foreign gene, called a trans-gene, is placed into an organism, in a process called transformation. A common problem for researchers is to determine quickly and easily if the target cells of the organism have actually taken up the transgene. A marker allows the researcher to determine whether the transgene has been transferred, where it is located, and when it is expressed (used to make protein).

Green fluorescent protein (GFP) serves as a visible marker in these mouse cells.

The marker itself is also a gene. It is placed next to the transgene to make a single piece of DNA, which is then transferred. Markers are chosen because their gene products (proteins) have obvious effects on the pheno-type of the organism. If the system is constructed properly, detection of the marker's product indicates that the transgene is present and functioning.

Marker systems exist in two broad categories: selectable markers and screenable markers. Selectable markers are typically genes for antibiotic resistance, which give the transformed organism (usually a single cell) the ability to live in the presence of an antibiotic. Screenable markers, also called reporter genes, typically cause a color change or other visible change in the tissue of the transformed organism. This allows the investigator to quickly screen a large group of cells for the ones that have been transformed. Selectable and screenable markers are essential to genetic engineering in both prokaryotes and eukaryotes, and are often built into engineered DNA plasmids used for genetic transformation.