<unk> s are a tool commonly used by molecular biologists to deliver genetic material into cells. This process can be performed inside a living organism (in vivo) or in cell culture (in vitro). Viruses have evolved specialized molecular mechanisms to efficiently transport their genomes inside the cells they infect. Delivery of genes by a virus is termed transduction  and the infected cells are described as transduced. Molecular biologists first harnessed this machinery in the 1970s. Paul Berg used a modified SV40 virus containing DNA from the bacteriophage lambda to infect monkey kidney cells maintained in culture.
Key properties of a<unk> <unk> s are tailored to their specific applications but generally share a few key properties.
Safety. Although<unk> s are occasionally created from pathogenic viruses, they are modified in such a way as to minimize the risk of handling them. This usually involves the deletion of a part of the<unk>  genome critical for<unk>  replication. Such a virus can efficiently infect cells but, once the infection has taken place, requires a helper virus to provide the missing proteins for production of new virions.
Low toxicity. The<unk>  should have a minimal effect on the physiology of the cell it infects.
Stability. Some viruses are genetically unstable and can rapidly rearrange their genomes. This is detrimental to predictability and reproducibility of the work conducted using a<unk>  and is avoided in their design.
Cell type specificity. Most<unk> s are engineered to infect as wide a range of cell types as possible. However, sometimes the opposite is preferred. The<unk>  receptor can be modified to target the virus to a specific kind of cell.
Viral vector
Vector
Vector 
Capsid
Genetically modified organism
Xenobiotic
Transposon
Transfer RNA