Vaccines A live vector vaccine is a vaccine that uses an organism (typically virus or bacterium) that does not cause disease to transport the pathogen genes into the body in order to stimulate an immune response. Viruses expressing pathogen proteins are currently being developed as vaccines against these pathogens, based on the same rationale as DNA vaccines. The genes used in such vaccines are usually antigen coding surface proteins from the pathogenic organism. They are then inserted into the genome of a non-pathogenic organism, Unlike attenuated vaccines, viral vector vaccines lack other pathogen genes required for replication, so infection by the pathogen is impossible. Adenoviruses are being actively developed as vaccine vectors. Medicine delivery A strain of canarypox virus modified to carry feline interleukin-2 is used to treat cats with fibrosarcoma. Types Retroviruses Retroviruses are one of the mainstays of current gene therapy approaches. The recombinant retroviruses such as the Moloney murine leukemia virus have the ability to integrate into the host genome in a stable fashion. They contain a reverse transcriptase to make a DNA copy of the RNA genome, and an integrase that allows integration into the host genome. They have been used in a number of FDA-approved clinical trials such as the SCID-X1 trial. Retroviral vectors can either be replication-competent or replication-defective. Replication-defective vectors are the most common choice in studies because the viruses have had the coding regions for the genes necessary for additional rounds of virion replication and packaging replaced with other genes, or deleted. These virus are capable of infecting their target cells and delivering their viral payload, but then fail to continue the typical lytic pathway that leads to cell lysis and death. Conversely, replication-competent viral vectors contain all necessary genes for virion synthesis, and continue to propagate themselves once infection occurs. Because the viral genome for these vectors is much lengthier, the length of the actual inserted gene of interest is limited compared to the possible length of the insert for replication-defective vectors. Depending on the viral vector, the typical maximum length of an allowable DNA insert in a replication-defective viral vector is usually about 8–10 kB. While this limits the introduction of many genomic sequences, most cDNA sequences can still be accommodated. The primary drawback to use of retroviruses such as the Moloney retrovirus involves the requirement for cells to be actively dividing for transduction. As a result, cells such as