The retroviridae are viruses that possess the unique ability to transcribe their genomic RNA into a DNA copy through a process called reverse transcription (1,2) (Figure 1). In infected cells, the DNA intermediates (unintegrated provirus) become stably associated with the host chromosome via an as yet uncharacterized integration process to form the provirus (3,4). These viruses, some of which produce malignant disease in host animals (see below) can be horizontally or vertically transmitted and have been studied intensively (3,5,6). Until very recently, rigorous studies of the structure of the genomes of these viruses were hampered by lack of sufficient quantities of intact viral RNA and restricted to those few viruses produced in abundance in tissue culture. With the advent of molecular cloning, these limitations are eliminated and all of the molecular biological techniques developed for analyzing DNA can now be easily applied to cloned proviral DNA. These applications include: 1) S1 nuclease analyses of hybrids between in vivo transcribed mRNA and proviral DNA (7) for identifying splice sites in cellular transcribed RNA species; 2) in vitro transcription (8,9) from proviral DNA for identifying promoter sequences; 3) hybrid-arrest protein synthesis to assign polypeptide products to specific regions of the genome (10);