A reporter enzyme is a gene that encodes an enzyme whose activity can be easily measured. For example, the gene encoding β-galactosidase can be used as a reporter gene. If the foreign DNA has been successfully integrated into the host genome and is being expressed, the β-galactosidase enzyme will also be expressed and will be able to cleave a colorless substrate, such as X-gal, into a blue product. Therefore, the presence of blue colonies indicates that the foreign DNA has been successfully integrated and expressed.
By using both a selectable marker and a reporter enzyme, it is possible to select for cells that have taken up foreign DNA and confirm that the DNA has been successfully integrated and expressed. This approach is commonly used in molecular biology to study gene expression, promoter activity, and protein localization, among other things.