The Vector or Viral method is similar to the Agrobacterium method in many ways, but the carrier of the engineered DNA is a virus. In the vector method, scientists prepare DNA containing the desired genes and insert this section into the DNA of the virus. Care must be taken, however, to remove the viral part of the virus' DNA, or else the virus along with the desired genes could be spread to the target organism. 

            The new viral genome is now ready to be put into the target cell. The virus with the new genome is mixed with the cells and the virus infects the host cells, inserting its DNA inside. The cell begins to express the new sequence, much like with an ordinary virus, but this time the sequence expressed contains the special engineered portion that codes for the desired genes. This gene can now be passed on to future cells, spreading the trait. 

            Marker genes are included on the engineered DNA, to give scientists the ability to determine which cells were in fact infected with the new viral genome. This marker gene is generally a gene for antibiotic resistance. The cells can be grown on selective media, only allowing those that were transformed by the virus to survive. This method is somewhat unpredictable, but clever, as it utilizes the natural process of a viral infection to spread a desireable gene in cells.