Transforming DNA viruses
(e.g., adenovirus and papovavirus) carry their own genes that are designed to stimulate
cell growth and proliferation. This is so that the host cells can support viral
growth and replication. These genes are said to be oncogenic, and as such they are
oncogenes.
Transforming retroviruses, on the other hand, accidentally
acquired genes from the infected host cells, and in doing so mutated these genes.
Since these cellular genes were proto-oncogenes, they became oncogenes in their mutated
forms.