Tumor-inducing viruses occur in several taxonomic groups. All RNA tumor viruses belong to retrovirus family, but the DNA tumor viruses come from several different groups. Their oncogenic potential is associated with their replication strategy, and in a vast majority of cases oncogenic transformation occurs only if the viral life cycle is aborted. The oncogenic phenotype results from discrete changes in the expression of key cellular control genes: oncogenes and tumor suppressor genes. Most often, the retroviruses cause the activation of oncogenes, and DNA tumor viruses usually target tumor suppressor genes.