The Retroviral Life Cycle and the Molecular Construction of Retrovirus Vectors

The discovery of a filterable agent that allowed the transmission of cancers in chickens (1 ) was the first identification of the viruses now known as retroviruses. Subsequently, genes transmitted by some retroviruses were identified as transforming oncogenes. These findings suggested that retroviruses may be used as genetic vectors, since retroviral oncogenes (v-onc ) are altered forms of “highjacked” normal cellular genes (2 ), and the retroviruses that transform cells in culture are often defective for replication because the v-onc genes have been substituted in place of one or more of the essential replicative genes (3 ). Such defective oncogenic retroviruses can be propagated only in the presence of a wild-type “helper” virus, which supplies the functional gene products of the virus. Retroviruses can now be modified to become vehicles for the delivery and expression of cloned genes into a wide variety of cells, for both experimental and therapeutic purposes.