Oncoviruses come in two different forms: viruses with a DNA genome, such as adenovirus, and viruses with an RNA genome, like the Human T-cell Leukemia viruses and several viruses known to be common in cats, mice and chickens.
Some oncogenic retroviruses, cancer-causing viruses with RNA genomes, incorporate their genome into the host cell using reverse transcriptases to make DNA. This DNA gets inserted into the cell DNA along with powerful promoter sequences (LTRs) that promote transcription of the viral DNA to reproduce more virus. However, sometimes the viral DNA incorporates a section of the host DNA which contains genes for growth promotion. These growth genes, sometimes called proto-oncogenes in their normal state, become oncogenic once incorporated into the viral DNA because of the increased transcription caused by the viral LTRs. This causes increased growth of the infected cell, leading to cellular proliferation and the formation of tumors. Numerous oncogenes have been discovered in the genomes of transforming retroviruses.
Other oncogenic retroviruses transform cells by integrating into the host gene near a proto-oncogene. If the viral LTRs are close enough to that oncogene, they will upregulate transcription not only of the viral DNA but of the proto-oncogene nearby, causing growth, cell proliferation, and tumor formation.