l RNA tumor viruses have become increasingly utilized in studies of cellular transformation and gene regulation. The genes of retroviruses exist in two forms; as extrachromosomal, RNA-containing, infectious particles and as DNA pro 2 viruses stably associated with cell genes. Components from the extracellular form can be collected in large quantity and purified for the preparation of molec ular probes. These probes can be used to dissect the sequence of events required for the establishment and expression of the integrated form. Furthermore the 2 genomes of retroviruses originated from normal...