Enzyme markers

• Enzyme markers can also be used, such as the gene that produces the lactase enzyme. Lactase will turn a particular colourless substrate blue.
• Similarly to the fluorescent marker genes, the desired gene is transplanted into the gene that produces lactase. If a plasmid from the required gene is present in a bacterial cell, then the colonies grown from it cannot produce lactase. Therefore, these bacteria will be unable to create the distinctive blue colour change.
• The bacteria that don't change the substrate colour should be kept and cloned because they contain the desired gene.