Introduction

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-one)
are altered forms of “high-jacked” normal cellular genes
(2), and the retroviruses that transform cells in culture are often defective for replication
because the v+,ncgenes 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.