Retroviruses are RNA-containing viruses using reverse transcriptase to produce DNA copies capable of insertion into host chromosomes. Appropriate genes are required to confer transforming ability to retroviruses. The src gene, a 60,000-dalton protein with protein kinase activity, is required by avian viruses to induce sarcomas. Normal cells have a gene (sarc) similar to the src gene. Retroviruses with oncogenic properties can arise by recombining with genes on the host chromosome. Herpesviruses, adenoviruses, and papovaviruses have transforming properties residing in only a portion of the genome. Probably, only one to two genes are required for transformation, regardless of the complexity of the virus.