the vector. It is transcribed to produce the single-stranded RNA viral genome and is marked by the presence of the ψ (psi) sequence. This sequence is used to package the genome into the virion. Adenoviruses As opposed to lentiviruses, adenoviral DNA does not integrate into the genome and is not replicated during cell division. This limits their use in basic research, although adenoviral vectors are still used in in vitro and also in vivo experiments. Their primary applications are in gene therapy and vaccination. Since humans commonly come in contact with adenoviruses, which cause respiratory, gastrointestinal and eye infections, majority of patients have already developed neutralizing antibodies which can inactivate the virus before it can reach the target cell. To overcome this problem scientists are currently investigating adenoviruses that infect different species to which humans do not have immunity, for example, the chimpanzee adenovirus used as a vector to transport SARS-CoV-2 spike gene in Oxford AstraZeneca COVID vaccine. PEGylation of adenoviruses for gene therapy can help prevent adverse reactions due to pre-existing adenovirus immunity. Adeno-associated viruses Adeno-associated virus (AAV) is a small virus that infects humans and some other primate species. AAV is not currently known to cause disease, and causes a very mild immune response. AAV can infect both dividing and non-dividing cells and may incorporate its genome into that of the host cell. Moreover, AAV mostly stays as episomal (replicating without incorporation into the chromosome); performing long and stable expression. These features make AAV a very attractive candidate for creating viral vectors for gene therapy. However, AAV can only bring up to 5kb which is considerably small compared to AAV's original capacity. Adeno-associated viral vectors have been engineered to evade virus recognition by TLR9 receptors by incorporating TLR9-inhibiting genes into the vector. Furthermore, because of its potential use as a gene therapy vector, researchers have created an altered AAV called self-complementary adeno-associated virus (scAAV). Whereas AAV packages a single strand of DNA and requires the process of second-strand synthesis, scAAV packages both strands which anneal together to form double stranded DNA. By skipping second strand synthesis scAAV allows for rapid expression in the cell. Otherwise, scAAV carries many characteristics of its AAV counterpart. Plant viruses Plant viruses can be used to engineer viral vectors, tools commonly used to deliver genetic material into plant cells; they are also sources of biomaterials and nanotechnology devices. Tobacco mosaic virus (TMV) is