Lentiviruses are a special type of retrovirus. These viruses are able to infect non-dividing cells and can therefore be used for in vivo gene transfer. This relieves concern about random genetic integration, but it also means that the therapeutic gene is only temporarily active. A disadvantage of adenoviral vectors is that the host is producing an immune response. This kills the infected cells and leads to the production of antibodies, preventing further infection by the same virus.
Adenoviruses can infect a broad range of human cells.
Inserting the gene is only part of the problem. The vector must also contain a mechanism for activating the therapeutic gene, which is not automatic. Gene therapy must include a timing and regulatory device, or promoter. Promoters are difficult to place in a therapeutic vector because they are often very complex and sometimes quite large. In some cases therapeutic genes entered the cells as expected, but this was not always the case in the Mulligan experiments.