DNA vaccines work by injecting genetically engineered plasmid containing the DNA sequence encoding the antigen(s) against which an immune response is sought, so the cells directly produce the antigen, thus causing a protective immunological response. DNA vaccines have theoretical advantages over conventional vaccines, including the ability to induce a wider range of immune response types. Several DNA vaccines have been tested for veterinary use. In some cases, protection from disease in animals has been obtained, in others not. As of August 2016 no DNA vaccines had been approved in the US for human use. Research is ongoing over the approach for viral, bacterial and parasitic diseases in humans, as well as for cancers.