Final answer:
Researchers can use reverse genetics, reporter genes, and DNA probes to isolate a gene when the protein product has not been isolated.
Step-by-step explanation:
One method that researchers can use to isolate a gene when the protein product has not been isolated is reverse genetics. This technique involves starting with a specific DNA sequence and determining what phenotype it produces. Researchers can also attach known genes, called reporter genes, to the gene of interest to monitor its expression and location in the organism. Another approach is to use a single-stranded DNA probe labeled with a molecular beacon, which can hybridize with a complementary single-stranded nucleic acid in the sample to find the gene of interest.