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...