Term formerly used to designate a now obsolete subfamily of viruses (family Retroviridae) composed of the RNA tumor viruses that contain two identical plus-stranded RNA molecules. Subgroups are based on antigenicity, host range, and kind of malignancy induced (avian, feline, hamster, or murine leukemia-sarcoma complex; murine mammary tumor virus; primate oncoviruses). Like other retroviruses, they contain RNA-dependent DNA polymerases (reverse transcriptases). An important aspect of these viruses seems to be use of viral reverse transcriptase to make DNA that can be integrated into the DNA of the host cell and will replicate along with cellular DNA.retrovirus.