A gene that identifies genetically transformed cells
Genetic transformation procedures are only successful at modifying a fraction of the targeted plant cells. Marker genes are therefore needed to identify which cells acquired the new genes. The marker gene - usually an antibiotic or herbicide resistance gene - is placed beside the gene of interest on the same piece of DNA. The DNA containing both genes is used to genetically modify the plant.
Marker genes are usually genes that make genetically modified cells resistant to some kind of toxic substance. If a marker gene is a gene for herbicide resistance, genetically modified plant tissue will be treated with herbicide. This treatment kills all of the plant cells that were not genetically modified. Because the marker gene was fused to the gene of interest, the surviving plants should also possess the gene of interest.
After the genetically modified plant cells are identified, the marker genes no longer serve a purpose. Some strategies are under development that enable gene transfer without marker genes, or that allow researchers to remove marker genes after selection. Scientists are also developing alternatives to antibiotic or herbicide resistance marker gene systems.