Selectable marker

A selectable marker is a trait that can be used to differentiate organisms. It is commonly used genetic engineering to sort bacteria that have been modified from those that haven't. For example, a common selectable marker is immunity to ampicillin which comes from the gene beta-lactamase. Growth of E. coli is normally stopped by ampicillin, so the gene of interest is inserted into the E. coli along with beta-lactamase and the strain is exposed to ampicillin- the bacteria that survive and grow will be only the ones that have successfully integrated the target gene.