The structure of the integrated DNA form of the retroviruses is shown. Long terminal repeat (LTR) sequences that function to initiate and terminate mRNA transcription are shown flanking the protein coding regions. Protein coding regions are: Gag, virion core polypeptide; Pro, protease; Pol, reverse transcriptase (polymerase) and integrase; Env, viral coat protein (envelope) required for cell entry. The reading frames of the protein coding regions are indicated by the elevation of the boxes, for example, the Pro and Env coding regions are in the same reading frame but Pol and Gag are in different reading frames. kb, distance in kilobases.