The genome of a virus when it is integrated into the host cell DNA. In the case of the retroviruses, their RNA genome has first to be transcribed to DNA by reverse transcriptase. The genes of the provirus may be transcribed and expressed, or the provirus may be maintained in a latent condition. The integration of the oncogenic viruses, such as Papovaviridae and retroviruses can lead to cell transformation.
Dictionary of molecular biology. 2004.