In vivo gene transfer is the introduction of genes to cells at the site at which they are found in the body. Foreign DNA can be injected into the cell, or its entry can be facilitated by various chemical or electronic ways, but these methods aren't very efficient. One requirement for gene therapy is that sufficient amounts of corrective DNA are delivered to enough cells to be therapeutically beneficial. An ideal gene delivery vehicle or vector would be able to enter a large number of cells and integrate its DNA into the host's chromosomes. The principle of all these vectors is to remove the disease causing components of the virus and insert recombinant genes that will be therapeutic to the patient.
Retroviruses have a limitation because they are unable to infect non-dividing cells.
Richard Mulligan and his coworkers created a retrovirus capable of infecting cells and splicing a corrective gene into chromosomes. The problem with retroviruses is that scientists have no control over how many copies of the gene become integrated or where on the chromosome they insert. The vector's genetic load may be inserted into a different gene, disrupting its expression. It could also put a cell onto a path of cancerous growth if the gene integrates within the regulatory region of a gene responsible for controlling cellular proliferation. Lentiviruses are a special type of retrovirus.