For a gene cloning experiment, a researcher inserts the lacZ gene into a bacterial plasmid. The lacZ gene encodes the enzyme ß-galactosidase, which catalyzes a synthetic molecule called X-gal to form a blue product. X-gal is included in the plating medium. The restriction enzyme used by the researcher cleaves a recognition sequence within the lacZ gene. Why did the researcher insert the lacZ gene into the plasmid?