Genome segments from bacteria and bacteriophages are dissected with restriction and other endonucleases. The segments are fractionated by physical methods (gel electrophoresis), and by sequence homology with specific RNA or DNA. Specific segments are then spliced to other genomes (plasmids, viruses, Bacillus subtilis transformation) for display of biological activity. The work till now has focused on the development of methodology, and will now turn to the development of DNA-sequence reagents as probes of cellular specificity.