Finally, it should not be toxic or immunogenic. Such a vector does not exist and none of the DNA delivery systems currently available for in vivo gene transfer is perfect with respect to any of these points. Gene therapy and the means to promote it depend heavily on the development and improvement of new gene vector systems. Most approaches to gene therapy use viruses as vectors. The virus's harmful genes are removed, and a therapeutic gene is inserted in their place.
Retroviruses are the first and most used vectors in human gene therapy.
They can infect important cellular types of diverse animals and humans with a high efficacy of infection. Retroviruses, particularly a disabled mouse virus, are proven to integrate and replicate the genetic material of interest in the genome of the host cell. Their disadvantage is that the integration of the viral genome only occurs during cell division, and they are not effective in tissues in which cells are not dividing. The integration of the viral genome into the cell creates an insertional mutation, and there is a possibility that a gene controlling cellular division may be affected leading to a cancerous growth. Another well-developed vector is adenovirus that can be used to insert new genes into many kinds of human cells and provide effective gene expression.