The lacZ gene encodes beta galactosidase which is responsible for converting X-Gal into a blue color. This enzyme has been used as a reporter of expression from the promoter region of genes cloned in a plasmid. When the plasmids containing the cloned genes are transformed into bacteria, the presence of the plasmid is detected by the activity of the beta galactosidases. If the plasmid contains no insert (no DNA), then no enzyme will be produced and no blue color will appear when cells are stained using X-gal.