pBR322 has genes for resistance against two antibiotics (tetracycline and ampicillin), an origin of replication and a variety of restriction sites for cloning of restriction fragments contained through cleavage with a specific enzyme. Foreign DNA is inserted at a site located in one of the two genes for resistance against antibiotics so that it will inactivate one of the two resistance genes. In plasmid pBR322, the gene conferring resistance to ampicillin (ApR) can be interrupted by the insertion of a DNA fragment into the PstI site, and the gene conferring resistance to tetracycline (TcR) can be interrupted by the insertion of a DNA fragment into the BamHI site. If we ligate a foreign DNA at the BamHI site of tetracycline resistance gene in pBR₃₂₂ the recombinant plasmid will not grow on tetracycline containing medium.